Ash

Ash is Marceline's ex-boyfriend, and appears in "Memory of a Memory". Marceline broke up with him because he sold Hambo her favorite teddy bear. Disguised as Rag Wizard, he used Finn & Jake to get ahold of Marceline's memory core containing her memory of dumping him so that he could destroy it, effectively making it like the memory never happened for her. As Marceline wakes up he and Marceline go to his house where he asks Marceline to make him dinner then she finds out what happened from Finn and Jake, Marceline comes out of his house with Finn and Jake and they start to beat him up while Jake stretches to a giant size and stomps on him.

Rag Wizard
"Rag Wizard" is a fake personality Ash came up with to trick Finn and Jake. This disguise has a beard and at the end it is tied. His clothes are made of rags, making him look more like a hobo than a wizard. He allegedly had no arms, and thus used his feet to interact with things. He claimed that he was Marceline's "spirit animal".

Trivia

 * Ash has a similar hair style to Marceline in the episode Go With Me?.
 * Adam Muto confirms that he is a wizard, not an undead creature.