Ricardio was revealed to be Ice King's own sentient heart in "Ricardio the Heart Guy." It literally left his body to pursue its own agenda. It seems to share the same passions as the Ice King, including the fixation on marrying Princess Bubblegum. Naturally, the Ice King is desperate for Ricardio to return to the inside of his body but Ricardio has no empathy for the Ice King, even beating him up and throwing him into a dumpster to die. Without his heart, the Ice King is virtually, but not completely, helpless, and will likely die without it after a time. In "Lady & Peebles" Ricardio escapes once again taking Ice King's internal sinews to fashion a body without regard for Ice King's well-being. Ricardio also calls the Ice King "Simon" in the episode "Lady & Peebles, revealing that Ricardio somewhat remembers Ice King's past life.
Ice King first interacted with Neptr in "What is Life?" when Finn tricked the Ice King into zapping Neptr with ice lightning so that the robot could become fully functional. After this, Ice King explained to Finn that, in zapping Neptr with his Ice Powers, the robot became infused with the Ice King's "private particles." To his mind, this made Neptr his "son." As a result, he wanted Neptr to choose between him and Finn. Neptr showed affection for the Ice King, calling him "papi," but chose to stay with Finn, whom he calls "creator," in order to serve his original purpose of throwing never-ending pies.
The Ice King and Neptr interacted with each other again in "Mystery Dungeon," where they worked together to escape the eponymous dungeon, along with Tree Trunks, Shelby, and the Earl of Lemongrab. In the episode, it is shown that the Ice King no longer remembers who Neptr is and had mistaken him for BMO. However, by the end of the episode, the two appear to have reestablished a relationship, with Neptr trying to cheer up his "papi."
In "Play Date" Finn and Jake invite Abracadaniel over to hang out with Ice King because they were tired of babysitting him. It was an immediate friendship. They were seen hanging out in a very childlike form, even admitting to the fact that they were now best friends. Additionally, they are seen doing normal sleepover things, such as putting on a show/play, and sneaking around the house after everyone else had gone to bed. Later in the episode they mess with Finn's Demon Sword and then release Kee-Oth and then threatens to kill them unless Finn returns his blood to him. After Finn returns the blood he kidnaps Jake mistaking that he is Joshua then releases them. After they head over to the Ice Kingdom revealing that it is rebuilt after Flame Princess burned it down in "Frost & Fire." It is also apparent that they are still extending their time together, and may even still be seen with each other in future episodes.
In "Thanks for the Crabapples, Giuseppe ," Ice King and Abracadaniel are still close, and go on a road trip together to create a new school of magic. Ice King even lets Abracadaniel use his crown to create an ice road for the bus they are using when he becomes too tired to use the magic himself. Ice King is also shown to be close to a number of other misfit wizards and magic creatures, including Ron James , the Ancient Sleeping Magi of Life Giving , and Little Dude .
In "Friends Forever" it revealed that they are no longer friends. Ice King is now currently keeping him captive in his house and it is not likely for them to be friends again.
Gunter and the other penguins
The Ice King's moodiness in life is expressed toward them. He will portray paternal feelings towards them, often referring to himself as "daddy" in the third person, and can even be understanding to their childlike antics, such as when Gunter was breaking bottles in "Still." However he also treats them as slaves, often seen yelling, abusing, and in "Five Short Graybles" he threw several of them in the garbage and marooned Gunter on a drifting iceberg for his own selfish reasons. Though annoyed, Gunter came back, so the penguins seem to be quite loyal. In "Princess Monster Wife" they are also shown to act in fear of Ice King. He even said "clap harder or I'll smack your buns." In "Reign of Gunters," he is the only one who could stop Gunter from causing terror in the Candy Kingdom. Sometimes, Ice King can do favors for the penguins, as he allowed them to lick his tears (except for Gunter, who Ice King states has a low-salt diet), as seen in "Five More Short Graybles," and dance for their entertainment in "Another Five More Short Graybles."
In "The Party's Over, Isla de Señorita," Ice King washes up on the shore of an island, later revealed to be the Island Lady. Having chosen to move on from Bubblegum, Ice King stays on the Island Lady and builds himself a house from the materials on the Island Lady. Due to the time Ice King spent with the Island Lady, the two have bonded as friends.
With the two having similar problems (the problems being their love interest not spending time with them, even though Bubblegum is not Ice King's girlfriend), Ice King and the Island Lady are seen talking about this matter. Due to both's interactions, they inspire each other to break up with their love interest. According to Ice King, the Island Lady was as much as an inspiration to him as he was to her.
Princess Bubblegum is Ice King's main love interest, but the relationship is one-sided. Since Finn had a crush on Princess Bubblegum before meeting Flame Princess, this made it even harder for Ice King to maintain any relationship that will last with her. Finn and Ice King would usually be seen fighting for the affections of Princess Bubblegum in earlier episodes. It seems that he also captures her more than any other princess and enjoys being near her.
In "Mortal Folly," he even tries to get her attention and marry her, and even tells her that he loves her. However, after she temporarily turns thirteen, he is not interested in her. He appears to be interested in her again when she turns 18 again as shown in "Wizard Battle," where he tries to win the competition by cheating in order to get a kiss on the lips from Princess Bubblegum. In "Holly Jolly Secrets Part II," however, it has been shown that Princess Bubblegum and the Ice King are capable of being civil, even hanging out, presumably when Finn is around to prevent anything bad from happening. The three were shown wearing Christmas sweaters, drinking cocoa, and watching tapes contentedly.
In "What Have You Done?" he tries to be kind to Princess Bubblegum by making it "snow" (scratching his beard so his beard flakes fall) but instead makes everyone sick with Freezer Burn Flu. When Princess Bubblegum goes to him for help he denies it and thinks she is hitting on him, making her upset. However, she just regards him as "an old fool."
At the end of "Lady & Peebles," Princess Bubblegum creates a new heart for Ice King after rescuing him from the dungeon where Ricardio was. Although Princess Bubblegum dislikes Ice King, she does not wish for him to die and even said "please" before asking him to stop being annoying and leave.
In "A Glitch is a Glitch," Ice King "reminds" Princess Bubblegum about how she told him that she would go on a date with him if he was the last person in Ooo, but she replies that she never really said that. He later reveals that he created a computer virus that would delete everything except him and the princess. Later in the episode, Princess Bubblegum tells him that even if she actually said that, he would not be the last person in Ooo because she would be still there and starts kissing her own hand, revealing that she would rather be in love with hand rather than the Ice King.
In "The Party's Over, Isla de Señorita," Ice King tries to kidnap Princess Bubblegum again when she was sleeping. After commenting how pretty the princess looks when she is sleeping, he puts the duct tape over her mouth, causing her to wake up with a muffled protest. She rips off the tape and immediately calls in the Banana Guards, who takes the Ice King out of the Candy Kingdom. Ice King realizes, that based on her reactions, that she does not love him, and decides to travel out far in Ooo to start somewhere "fresh." Ice King describes their relationship to the Island Lady, though they actually are not, and connected with her relationship with the Party God through this. He tells her that Princess Bubblegum is so "closed off" to her emotions that she crushes the relationship so that she does not ever have to develop emotions as he thinks that he and Bubblegum have a real, romantic relationship.
At the end of the episode, he officially broke up with Princess Bubblegum. He tells her that he is breaking up with her because she treats him badly and is always away at parties, which closely resembles the relationship between the Island Lady and the Party God. However, the princess does not seem to care about this and tries to call in the Banana Guards. Irritated by her reactions, Ice King yells, "Banana Guards yourself, Princess!" in Princess Bubblegum's face and leaves, much to Bubblegum's confusion.
Aside from Princess Bubblegum, Ice King has shown an obsession with all the other princesses. His most common is Wildberry Princess, for unknown reasons. In "Hitman," Ice King states that Lumpy Space Princess and Ghost Princess are his least favorite princesses. This is due to the fact that he cannot hold Ghost Princess with his "love mitts." He also dislikes Lumpy Space Princess as he prefers smooth princesses, as stated in "Loyalty to the King." Although later in the episode, even when she punches in her lumps, he still rejected her so it seems he finds her personality annoying.
Fionna is a gender-swapped Finn created by Ice King for a series of fan fiction novels he has written involving adventures of her and her cat Cake. While generally fan-fictions, Ice King has generally turned himself into a Mary Sue character where Fionna is madly in love with him and the duo go on adventures together while confessing their love for each other.
While his creation, Ice King has obsessed in believing that Fionna is real and is willing to do anything to bring her to life; even attempting to bring a book of his to the Ancient Sleeping Magi of Life Giving, who in turn brings the book itself to life instead of creating a living, breathing Fionna. Regardless of the set-back, Ice King continues to believe that Fionna will be brought to life someday, all the while being called "delusional" by everyone. He also has a secret room with ice statues of her and Cake along with hundreds of fan fictions deep in his castle.
Ice King mentioned to Party God that they both met once at one of Peppermint Butler's parties, but Party God had forgotten. Having chosen a different path, the Ice King spent quite a time with the Party God's girlfriend, the Island Lady.
Ice King was led to believe that the Island Lady should break up with the Party God because he does not pay attention to her at all and continues partying. But with the Island Lady too afraid break up with him, Ice King tried to break up with him for her. However, there is a misunderstanding, and the Party God is led to believe Ice King is flirting with his girlfriend. Thus, a fight begins between them. Ice King won the battle and sent the Party God into space.
Finn and Jake
At the beginning Finn and Jake were Ice King's enemies (animated short, "Prisoners of Love," "What is Life?"), but already in "Prisoners of Love" Finn says "I don't know how to help you!." In "Ricardio the Heart Guy" they make first alliance.
In "When Wedding Bells Thaw," Ice King almost kills Finn and Jake by mistake, but then he asks them to throw him a "man-lorette" party. Later, when Finn and Jake sabotage Ice King's wedding ceremony, through certain complications he ends up accidentally temporarily marrying Jake.
In "What Have You Done?," Finn and Jake free Ice King, which makes him think they are his friends, and then they pretend to really be them. In "The Eyes" Ice King disturbs them so much that they threat to kill him and in "Loyalty to the King" they three are opponents again. But from "Mortal Recoil" on, Ice King frequently tries to be friends with Finn and Jake in his own misguided way and has willingly made alliances with them.
In "Hitman," the Ice King makes it clear that he does not want to kill Finn or Jake—the most he would do is "punch [them] in the gut and make [them] spit up [their] lunch." This episode for the first time suggests that Ice King has a mixed view on Finn and Jake. The three are essentially friends, but since Ice King constantly kidnaps princesses and the two would have to save them, Ice King gets frustrated that Finn and Jake stopped him from his motives, and Finn and Jake would consider Ice King as a foe and believes that every motive he does is evil, since this is what they always see from him. This is one of the main reasons Ice King usually has negative feelings towards Finn and Jake. Though Ice King's actions rarely have malicious intent, Finn and Jake must often step in to stop him from kidnapping princesses.
Finn and Jake often misinterpret Ice King's actions as being part of some evil plan, such as his throwing out his "evil" collection of tapes in "Holly Jolly Secrets Part I" and going to see Marceline about writing a song in "I Remember You." In "The Chamber of Frozen Blades," they sneak into his house and wind up breaking most of his stuff in their effort to find out about Ice King's plan to kidnap a princess, wherein in actuality, he had only just taken a pregnant Gunter to the hospital, though he does wind up kidnapping Doctor Princess while there.
In "Holly Jolly Secrets Part II," the Ice King is shown to have a locket with the pictures of Finn and Jake. Near the end of the episode, the three are shown snuggled up side-by-side by a roaring fire, wearing big Christmas sweaters.
In the episode "All the Little People," Finn does not put Ice King in the "C-list"er category, that characters such as Xergiok and Donny are in. Finn also mentions that Donny is "alright," which implies that Ice King is more than "alright" to Finn. In "Princess Potluck," Ice King seems to think that Finn and Jake are his "BFs," though this is most likely the result of his skewed view of the world.
In "Earth & Water," after his kingdom was destroyed by Flame Princess in "Frost & Fire" and while his penguins rebuild, Ice King stays with Finn and Jake. In "Play Date," Finn is shown to have a great deal of empathy for Ice King, even to the point of enjoying his company. While Finn finds Ice King to be an interesting companion, Jake has no tolerance for his antics. Jake has to go as far as outright demanding Finn to get rid of Ice King. He has disdain for all of Ice King's oddities, even his attempts of affection. Finn, on the other hand, finds his efforts endearing and does not mind his presence in their house.
In "Frost & Fire," when he taunt (which is an unlike thing he does with princesses) Flame Princess for hugging a log, she beats him up and kicks him back into the Ice Kingdom. Since then, Ice King developed a fear on her and is willing to give anything to her to placate her. Later in the episode, Finn forged an insulting letter to make the two fight again. Enraged, they had their final battle in the Ice Kingdom, which causes the destruction of it when Flame Princess effortlessly throws the Ice King down. After that fight, Ice King started to think that she is crazy and his fear surged, even cowering in fear when it was not even hinted that she was around him when he thought that she was in "Earth & Water."
In "The Cooler," he was seen to be helping Bubblegum in freezing the Fire Kingdom's core, indicating that he might had held hatred for her for destroying his Kingdom.
There is no direct relationship between The Ice King and BMO, although in "Holly Jolly Secrets Part I" he recognizes BMO's alarm. In "All the Little People," tiny BMO and tiny Ice King appear to have a relationship. Though, due to the random jokes often found in the series, this may not prove anything. The Ice King knows BMO's abilities as a computer, and confuses BMO with Neptr in "Mystery Dungeon." In "President Porpoise is Missing!", they are seen playing submarines together. They became doormans together in "Always BMO Closing".
Ice King and Tree Trunks never interacted with each other until "Mystery Dungeon" where they will be working together, along with Neptr, Earl of Lemongrab, and Shelby.
Ice King and Shelby never interacted with each other until "Mystery Dungeon" where they work together, along with Neptr, Earl of Lemongrab, and Tree Trunks.
Earl of Lemongrab
Ice King and Lemongrab never interacted with each other until "Mystery Dungeon" where they work together, along with Neptr, Shelby, and Tree Trunks. Lemongrab attempts to eat the Ice King twice over the duration of the episode, and attempts to kill him once for "having no purpose."
People from his Past as Simon Petrikov
The Empress and Ice King apparently used to know each other, and Empress was a sort of twisted love interest. In the Adventure Time eight part miniseries "Stakes", they show in the episode "The Empress Eyes" that at some point after Simon left Marcy knowing he was going to become too dangerous and messed up to take care of her, he ended up being hypnotized by The Empress and became a slave to her. He was apparently completely in love with her, and would do anything for her. The Empress saw him as a pawn, but at least admitted there could be something to like about him by saying he had a "kind of silver fox thing going on." But her opinion of the current Ice King is completely negative. She thinks he's basically an idiotic train-wreck and she especially hates him because at this point he is "too stupid to fall under her spell." But despite Ice King being unable to fall under her spell, he still followed her orders in the hopes of getting in a relationship with the Empress. The Empress also told Marceline while fighting how she thinks he was happier back then when he was with herself, which angered Marcy.
Betty was a human woman revealed to have once been the fiancée of a man named Simon Petrikov, who would much later become Ice King. She and Petrikov had a very committed relationship until Petrikov obtained an enchanted crown, which gradually began to alter his mind and body. When Petrikov put on the crown for the first time, something he did terrified and angered Betty and caused her to leave, as evidenced by Simon saying that "she looked at [him] with such contempt." Simon never saw her again, and he records on video his desperate wish to regain his sanity so that she might one day return to him. It is not known if she was killed during the Mushroom War, though most likely due to Simon's note in "I Remember You" saying that he and Marceline may be alone in the world after the apocalypse of the mushroom bomb.
In the episode "Betty," a briefly restored Simon creates a time portal to go back and apologize to Betty. He encounters her shortly after he has put on the crown and is terrorizing his "princess" with his ice powers. Betty is astonished to learn she had left Simon, but realizes the reason he never saw her again is because she jumped through the time portal to be with his future self, which she does. After defeating the anti-magic creature Bella Noche, whose neutralizing of the Ice King's crown had turned him back into Simon (and left him dying, since the ice crown's magic kept him alive but insane for all those years), Betty briefly watches the restored Ice King from a flying carpet before heading off, determined to find a way to turn him back into Simon without his dying from lack of magic. It is clear that Betty still loves Simon and is devoted to him, but he is unable to remember her in his Ice King form.
As revealed in the flashback in "Bespoken For", Ice King becomes surprised when the now wizard Betty (warped by the power of Magic Man's hat) invites him out to dinner in hopes of trying to restore him. Despite things not going the way as planned, Ice King gains a new respect for Betty, referring to her as "Weird Lady" when he, Finn and Jake try to save her from Patience St. Pim's captivity after she uses her power to turn Ooo into a world based on the four magical elements.
In "I Remember You," Ice King is revealed to have known Marceline in the past during the aftermath of the Mushroom War. He is also the one who gave Marceline her old toy, Hambo. Although as much as Marceline remembers Ice King as Simon, he does not seem to remember her as much as she explains to him. It is implied through Marceline's dialogue that Ice King has repeatedly sought her out at her different residences over the years, (This alludes back to "Evicted!" where it is revealed that Marceline switched back and forth with her many homes to hide from Ice King when she did not want to deal with him) but does not remember their past friendship or his life as Simon, something that angers Marceline and sometimes even drives her to tears.
In "Simon & Marcy," depicts this period of their lives, where Simon worked hard to keep young Marceline safe and happy in the devastated post-Mushroom War world. In the flashback, Simon is shown to look for food for Marceline, entertain her with jokes, plays and songs (possibly inspiring her own interest in music) and risking his life to find her some chicken soup when she is sick. However, it is also shown that Simon occasionally needed to put the ice crown on his head to access its powers to protect Marceline, and each time he did so accelerated his transformation into the Ice King, with longer hair and a longer nose, and a crazed, nonsensical personality. Marceline was aware of the crown's effect on him and made him promise not to put it on, but Simon was forced to in order to save them from some mutants, and was only barely able to get the crown off. Afterwards, he refers to her as "Gunther," indicating his transformation had continued.
A letter he wrote for her in the past indicates that Simon stayed away from Marceline because he was aware the crown's powers were sapping his sanity (and keeping him alive) as he turned into the Ice King, and that he was afraid of saying or doing something harmful to her based on recent events. He remains a fan of her music even as the Ice King, and irritates her by showing up at her doorstep to write a song to help him get princesses. However, despite her frustration with Ice King's behavior, Marceline maintains some affection for him, turning away Finn and Jake when they offer to wallop him for her, and eventually performing the duet "Remember You" with Ice King based on their past relationship. She also admits in her song "Nuts" that while she is deeply annoyed with Ice King, she is always glad to see him when he shows up. In addition the Ice King seems to still hold some concern for Marceline, immediately apologizing for shoving her and then hides on top of the refrigerator, showing more guilt than he does when doing much worse things against Finn or the other people of Ooo.
And in the episode, "Finn the Human," Farmworld Marceline is seen as an older woman, saying to Farmworld Finn that her beloved friend Simon Petrikov stopped a missile using the magic from his crown, then pointing to his skeleton under a frozen missile also stating that he could not save himself. Events surrounding the full nature of their past relationship have yet to be seen in the series.
Since the events of "I Remember You," Marceline has visited Ice King in his castle at least once, as shown in "Bad Little Boy." She also appears to be more tolerant of Ice King in this episode, showing that she forgives him for not remembering her. In "Simon & Marcy," a more 'in depth' relationship between them is explored. For example, Marceline invites him to play basketball. He is shown to have been like a father to her, being very protective of her, keeping his promises to her, and showing visible concern when she was ill. In the present, Marceline tells Finn and Jake that the Ice King "means a lot" to her, and that she loves him.