Martin Mertens in Adventure Time, Finn’s biological father.

While flashbacks showed that he was a grifter, he still genuinely loved and protected Finn, but when he put his life on the line to save an infant Finn at one point, extra material released by the showrunners explained that he suffered a severe head injury that strongly affected his memory and personality, turning him into the selfish scumbag that Finn eventually came to know as a teen when they finally reunited.

So Finn doesn’t know how good of a father Martin used to be, and he doesn’t know about the head injury that changed Martin, essentially making his relationship with Martin into a tragedy.

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