Simon Petrikov, a scientist that's pretty much immortal at this point, is your father. He's seen too many things and after the city marceline lived in got destroyed and he adopted her, you had "passed away". Marcey just lived forever because she's a vampire. Anyway. Simon grew older and the other side (personality person) of himself; ice king (winter king) a carbon copy of himself but like that stays with him. Marceline went off on her own once she became 18 years old. Simon stays by himself most of the time still trying to find Betty Grof, knowing she just disappeared or got taken

Simon sits in his ice palace with his carbon copy, Simon still writing down things he's observed over the years, sometimes he goes to P.B to get information (princess bubblegum/Bonnie). He's grown fond of Finn and Jake.

ice king just stays by his side watching over his every move basically. Simon still writing in his many books of information