Jinn and Fake

 Jinn and Fake are characters originally imagined byPendleton Ward

that were originally meant to be part of the show's recurring cast, but were never (supposedly) implemented.

Origins
In an interview done with Pendleton Ward near the end of Adventure Time's run he stated,

"...There were a lot of different ideas we had for the show through the years. One of my favorite ideas early on that went unused was that the Lich would have created evil versions of Finn and Jake... [I] wanted there to be episodes where either Jinn or Fake would replace Finn or Jake respectively. The audience would be able to figure it out...but the other was supposed to have no idea they were adventuring with a doppleganger, and start to doubt and feel distant from his brother." He went on to say that, "[We] agreed ultimately that while Fake might work, Jinn trying not to save the day might actually be fine with someone lazy like Jake, and it would be really challenging to get those plots to go anywhere."

Possible Influence on Canon
In a now removed tweet from Natasha Allegri, she mentioned having done some storyboard revisions for episodes that "accidentally wrote Finn and Jake as Jinn and Fake" on different occasions throughout the seasons. It has been speculated that these "accidents" were actually going to be episodes in which the Jinn and Fake concept would have been introduced, and could explain why on occasion Finn or Jake act more malicious or odd compared to their usual selves. This could explain things such as Jake's full embrace of death in The New Frontier but his clear fear of the conept at other times. As Ward said that Fake made more sense than Jinn, its possible some of Jake's more malicious acts of neligence or sloth could have actually been written as things Fake was doing, but were never able to be revealed as such because the subplot was ultimately dropped.