Ash is Marceline’s jerk ex-boyfriend, who is a magician, although not a very powerful one. In the TV episode “Memory of a Memory” Marceline originally breaks up with Ash after he sells her childhood teddy bear to a witch so he can buy a magic wand with real cherry blossoms.
Ash tricks Finn into removing the memory of Marceline breaking up with him from her mind, so that she thinks they are still together, but Finn later gives her his memory of removing the memory, which included the original memory because he viewed it before erasing it. Marceline then beats up Ash, (Finn gets a few kicks in too) effectively ending their relationship again.