|Introduced in||"His Hero"|
|Latest appearance|| "The Lich"|
"Jake the Dog" (corpse)
|Voiced by||Lou Ferrigno|
As described in "Billy's Song," when Billy was younger he was a hero of action much like Finn. He is known to have rescued Cotton Candy Princess from Fire Count, slain an evil ocean, and fought against a giant bear. Perhaps most importantly, he's also said to have defeated the Lich.
When Finn and Jake finally meet him, he is old (his red hair turned white), and he frowns upon violent heroism like theirs. He hated the futility of fighting evil when more would always pop up later, so he chose to help people indirectly through non-violence. He even had his own magical dog, though it is deceased by Finn's time. Finn and Jake try to follow his example of pacifism but they did more harm than good so they ended up returning to their violent ways, with Billy's eventual admiration.
He was in a relationship with an old lady who was introduced to Billy through Finn and Jake.
Prior to his appearance in the Season 4 finale "The Lich," after the events described in Billy's Song, the Lich, while possessing the snail, kills Billy and uses his body as a gruesome "costume" by wearing Billy's corpse. The Lich uses this disguise to trick Finn, as Finn trusts Billy. This was confirmed by Prismo in "Finn the Human."
Billy was very tall and had a long beard and long hair which looks like that of a caveman. His large and unusual hands contain six fingers including the thumb (may be an effect of the Mushroom War), though most Humanoids in Ooo possess four-fingered hands. He had blue veins bulging out on the back of each hand. Billy wore no shirt or shoes, and had brown pants.
He bore a strange scar that looks like he had stitches on his left shoulder. During his younger days he possessed a gauntlet with a jewel on the wrist and an eye on the top of the hand which he used for combat, including and most importantly during his battle with the Lich. He 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 can summon by calling its name. He was almost as big as the Lich.
- Billy is left-handed.
- 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." 
- Billy's name and heroic attributes may be a reference to the song "Billy Don't Be a Hero," originally recorded by Paper Lace but also made famous by Bo Donaldson and the Heywoods.
- 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 kingdoms (Ice King And Fire Count) and always capture Princesses, the Princesses most saved are from the Candy Kingdom (Princess Bubblegum and Cotton Candy Princess), both of whom gave the hero a piece of their hair, and both of them have magical dogs. They both keep piles of treasure in their homes too.
- Jake stated 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 6 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.
- Billy may have been a survivor of the Great Mushroom War, being his appearance and his long-time feud with the Lich.
- Billy is confirmed to be dead by Prismo in "Finn the Human" when he describes the Lich as "a ghost wearing a dead guy."