Billy was a legendary hero in the Land of Ooo who first appeared in "His Hero." Billy is idolized by Finn and Jake due to his reputation as a great warrior. However, by the time the two meet the hero, Billy no longer believes that violent heroism is the solution in stopping evil. Despite this, Finn and Jake are able to convince Billy that fighting evil is a good cause, gaining the hero's admiration and friendship.
In his youth, Billy had a reputation as a powerful warrior in the Land of Ooo. He was known for impressive feats, such as rescuing Cotton Candy Princess from the Fire Count, slaying an Evil Ocean, and fighting a giant bear. Around this time, Billy dated Canyon, and the power couple often went on motorcycle rides while fighting monsters. Billy's greatest accomplishment as a warrior was when he defeated the Lich before it could destroy Ooo with the unholy power gathered from the Earth's life force. The hero was then able to trap the Lich in an amber prison by pummeling it into the resin on the pinnacle of the Candy Castle.
Despite his fame and accomplishments as a hero, Billy began to believe that the task of fighting evil was futile as new evil would always arise later on. The hero soon developed a void in his being and lost hope in heroism. Instead, he began to favor nonviolent solutions to evil and was an advocate of active community involvement. He also developed a habit of watching movies and playing video games. In "Billy's Bucket List," Canyon explains that Billy's loss of belief was the reason why the two eventually severed their relationship. However, with the help of Finn and Jake, Billy gained renewed hope in the heroic cause and began to fight evil once again. The hero also developed a relationship with an old lady after meeting her through Finn and Jake.
Although Billy was a formidable force during his youth, his old age and long period of inaction may have contributed to his demise at the hands of the Lich during its return in "The Lich." After killing the hero, the Lich used Billy's body as a gruesome "costume" by wearing his corpse. The Lich uses this disguise to trick Finn into opening a wormhole with the Enchiridion. Billy's death was confirmed by Prismo in "Finn the Human" and later in "Billy's Bucket List."
Billy's carcass was destroyed by The Lich in "Escape from the Citadel."
In his youth, Billy was very strong and brave, as well as kind and righteous (much like Finn). However as Billy aged, he lost his sense of agency as his acts of heroism, in his eyes, failed to resolve anything in the grand scheme of the never-ending battle of good vs. evil. Overtime, he focused less and less on heroic feats and adventures creating a literal void in his heart. However after meeting Finn and Jake and becoming their friend, Billy 's void is filled, rekindling his love of being an adventurous hero.
Billy was a very tall humanoid with a long beard and long hair, making him resemble a caveman. In his youth, Billy's hair color was a bright red, but this grew white as the hero aged. His large and unusual hands contain six fingers including the thumb, though most humanoids in Ooo possess four-fingered hands. He had blue veins bulging out on the back of each hand, and his left shoulder bore a strange scar with stitches.
Billy wore no shirt or shoes and had brown pants. During his younger days, Billy possessed the Gauntlet of the Hero and used it extensively in combat. This weapon proved to be useful in aiding him during his battle with the Lich. In his dotage, Billy had a void in his chest where his heart would be until Finn's bold words fill it in "His Hero." He also appeared to have a magic sword called "Nothung" which he could summon by calling its name.
Finn idolized Billy and was amazed by his feats (as shown in Billy's bucket list, when Billy defeated a group of monsters). They also became very good friends and even had a slight father-son relationship. Billy also appears to believe in Finn. After Billy's death, Finn was slightly in denial about it and often gets traumatized about it as well.
Jake, like Finn, idolized Billy. Unlike Finn, however, Jake had a slight crush on him. After his death, however, Jake had a harder time accepting that it happened (as shown when he said he hoped that Billy and Canyon would get back together).
She and Billy were in a relationship before but broke up since Billy decided to focus less on being a hero. However, before they broke up, they were a power couple and often went on rides on Billy's motorcycle. At the end of "Billy's Bucket list," Billy says that he watches her sleep and says that "love is weird" (hinting that he may still care about Canyon).
The Old Lady met Billy after Finn and Jake had saved her from the Swamp Giant and brought her to show that they had saved her using violence. She expressed her joy to still be alive because of the heroes. Seeing this, helped fix the void in Billy's old heart; she was last seen perching on Billy's shoulder after shaking off Finn and Jake. Though he was surprised to see her on his shoulders, he seemed happy to meet her. She was seen hugging him as Finn and Jake run outside. In "The Lich", she and Billy are seen kissing in Finn's dreams. After awakening both Jake and Finn refer to her as Billy's "Lady", hinting she and Billy were in a relationship before his death.
The Lich was a deadly rival of Billy and were arch-enemies. The Lich and Billy share the same towering height and were on par in power. The Lich was defeated by Billy in the episode 'His Hero' by having his face kicked into a tower and trapped in amber as he was about to attack Billy with his green flames. After The Lich was freed by the hypnotized snail he controlled, he killed Billy in vengeance and wore his body like a costume, using the corpse of the hero to manipulate Finn and Jake to get to The Time Room. Finn and Jake still find the fact that Billy is dead something hard to accept.
- "Finn the Human" (Lich controls his body)
- "Jake the Dog" (Lich controls his body)
- "Wake Up" (Lich controls his body)
- "Escape from the Citadel" (Lich controls his body)
- "Crossover" (Farmworld Billy)
- Billy is left-handed.
- The Lich used Billy's carcass as a disguise to gain entrance to the Time Room after killing him.
- In earlier versions of the storyboard, Billy was named "Hogarth."
- Billy's sword is named "Nothung" (pronounced No-Tongue) after the sword belonging first to Siegmund and later Siegfried from Richard Wagner's cycle of operas Der Ring des Nibelungen (The Ring of the Nibelung). Billy has the ability to call his sword by saying its name. He retains the ability to do this even when he is old.
- Billy is voiced by Lou Ferrigno, who also played The Incredible Hulk in the live action TV series from the 1970s.
- Lou Ferrigno is partially deaf, which causes his speech to sound slurred. This resulted in Billy speaking with a lisp.
- Billy's appearance in the "His Hero" title card is based on Frank Frazetta's painting "Death Dealer." 
- On some concept art for Adventure Time, Billy can be seen breaking one horn off the Lich's helmet.
- Both Finn and Billy have many things in common: their arch enemies are rulers of elemental kingdoms (Ice King and Fire Count) and capture Princesses, the Princesses they've saved are from the Candy Kingdom, (Princess Bubblegum and Cotton Candy Princess) both of whom gave the hero a piece of their hair. They both keep piles of treasure in their homes. Both also have had a dog at their side, (Jake and the unnamed dog corpse in Billy's lair), and can both play the flute.
- Jake stated before that he has a secret crush on Billy.
- Billy has six fingers, while the show's characters are commonly portrayed with four fingers. His gauntlet also has six fingers. Considering his large stature and the number of fingers he possesses, Billy could possibly be a Nephilim, or at least a reference to the legend.
- Although Billy has 6 fingers on each hand, he only has 4 toes on each foot, making all of them to be 20 (the equal value as what humans possess - 10 fingers (5 fingers per hand) and 10 toes (5 toes per foot)).
- Billy is confirmed to be dead by Prismo in "Finn the Human," when he describes the Lich as "a ghost wearing a dead guy."
- He is the first recurring character to die.
- It is unknown how Billy knew about Finn's biological father, Martin.
- Billy was also shown trying to avoid telling Finn about Martin and only did so after Finn insisted, likely because he did not want Finn to know that his father was a criminal.
- Billy's demise might be a reference to Harvey Dent's quote from Christopher Nolan's "The Dark Knight": "You either die a hero or you live long enough to see yourself become the villain". To prove the connection further, the face of the Lich inside Billy's corpse it's very similar to Two-Face, the villain who Harvey Dent turned into.