Abraham Lincoln is the Ooo equivalent of Jesus and the King of Mars himself who was allies with Finn and Jake; telling Finn to believe in himself. He was originally deceased by trading his immortrality to Death but Wrath gave it back to him after cheating Death with the Easter Bunny Day dance.
In the episode "Best Friends", he was the friend of Hiro Sohma eating lunch with him, reciting his speeches and building a log cabin. He also appeared in "The Tribunal" serving as a judge for the Mars Court with Burger Barn and Mike Teavee as plantiffs. Noah Cooper disguised himself as Abraham Lincoln complete with an eye patch and a banjo and a good voice mimick to convince the Hiitachan twins to stop looking for the penny. It didn't work when Orochimaru came up dressed as George Washington.
Abraham Lincoln (King of Mars) is voiced by Adventure Time creator Pendleton Ward.