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 Reynolds Quotes

Mac: I browned out that evening.
Frank: Browned out? What's browned out?
Mac: It's when you drink so much that everything goes brown. It's not as sever as a black out because I remember bits and pieces. I call it browning out.

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?

It's Always Sunny Quotes

Naked pics online? That's disgusting. On a website? There's so many of them though. Where? Which one?

Mac

Dee: Who slams a door?
Frank: Babies.
Dennis: That guy has some real growing up to do. Have some repect for Christ's sake... I am legend.