Martin Mertens, also formerly known as Mr. M, is Finn's biological father. He is a chubby man with a long, dark blond beard, mustache, and a mostly bald head, except for a single, curling hair at the top. In our fanart Adventure Time Martin Mertens cursor pack, he is wearing a red leotard-type outfit with red spiked shoulders. Unlike Finn's personality, Martin is an incredibly selfish, lazy, ignorant exploitative, and a narcissistic individual who hardly acknowledges his son's existence unless it profits him.
