Frank Reynolds show up during the second season as Dennis and Sweet Dee's father. Frank was once a rich, successful business man, but after his divorce, wanted to come live like a slob with Charlie to experience life again. He forced his way into the gang by buying up the building. It was revealed that Frank is not the actualy biological father of Dennis and Sweet Dee, but might actually be Charlie's father.
Frank: I got my first kiss there. (stares off into the distance)
Frank (breaks down into tears): It was terrible. But not her. She was an angel. Always smiling... that's because she had no lips. But her mouth was still very much in play.
Therapist: Let's talk about the dishes.
Frank: She died two weeks later. She thought she was a spaceman with a plastic bag for a helmet.
Frank: I opened up to a therapist just once. I was a kid. I got into a fight. The doctor asked me question after question, got me so scrambled up. Next thing I know, I was shanghai'd upstate to a nitwit school. You know what a nitwit school is?
Therapist: I assume you mean a school for the mentally disabled.
Frank (spits pistachio shell): Yeah, not just for nuts in the head, but bodies, too. Back then science was real crude, they stuck us all together. My roommate was a frog-kid. You ever see a frog-kid?