Billy

Billy is a character who appears in "His Hero." Billy is Finn and Jake's idol and the most powerful hero seen in the series. Thus, the two are immense fans of him.

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 a 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 Finn's time. Finn and Jake try to follow his example of pacifism but end up returning to their violent ways, with Billy's eventual admiration.

In "Mortal Folly," it's revealed that the gauntlet which he wore when he was young is the only known weapon that can hurt The Lich.

Appearance
Billy is very tall and has a long beard and long hair which looks like that of a caveman. Each of his unusually large hands has six digits: 5 fingers and 1 thumb. Most Humanoids in Ooo possess four-fingered hands.

He bears 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 (one of the Cosmic Gauntlets in "Rainy Day Daydream" also has six digits, it may have been another of his weapons). He also appears to have a magic sword called "Nothung," which he can summon by calling its name

Quotes
See Billy/Quotes 

Trivia

 * In earlier versions of the storyboard, Billy was named "Hogarth."
 * Billy's sword is named "Nothung" 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.
 * Coincidentally, the actor who played the Hulk's human form on the show had a similar name: Bill Bixby.
 * Adam Muto has stated that Billy resembles a half giant, although that was just a guess, not a statement of fact.
 * 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.
 * It can be seen on some concept art for Adventure Time that Billy was the reason The Lich only has one horn on his helmet and the other horn was broken off during their fight.
 * The gemstone on Billy's headband appears to be similar to the one on Princess Bubblegum's crown, only a slightly different design. It might serve the purpose of protecting Billy from the Lich's mind control.
 * 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 ), and both of them have magical dogs. They both keep piles of treasure in their homes too.
 * According to Pendleton Ward, Billy will return and/or be alluded to in upcoming episodes.
 * Billy's gauntlet has six fingers, while the shows characters are commonly portrayed with four fingers.