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

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.

Mac: He doesn't have any poison.
Charlie: I don't have any on me, but I do keep some in my fridge at home in the relish jar.
Frank: There's poison in that jar? I thought I was allergic to pickles. What's in the jar with the skull and crossbones?
Charlie: Well that's mayonnaise. It's a decoy.
Frank: And the mayo?
Charlie: That's shampoo.
Frank: You're telling I've been putting shampoo on my sandwiches?
Charlie: If you've been using the mayonnaise, then yeah, probably.

It's Always Sunny Quotes

With real power comes real responsibility and I don't want any of that shit. I just want the money and the illusion of power. And puss.

Dennis

Dennis: I am having feelings again. Like some kind of fourteen year old kid. You remember, feelings right?
Mac: Yeah. I have feelings every single day of my life.
Dennis: Do you?
Mac: Are you saying you don't have feelings?
Dennis: What I'm saying is a built up a shell.. a shell around myself. A cold, calculated shell that couldn't be broken by anything but marriage.