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

Frank: (to Liam) I understand the McPoyle bloodline is very strong.
Liam: Legions of us, thousands sturdy, once ruled this fine land. Our bloodline was as pure as the driven snow.
Dee: Then what happened?
Liam: Syphilis killed about half of us.

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.

It's Always Sunny Quotes

Charlie: what is your spaghetti policy?
Dee: Are you hearing this? He doesn't belong in a place like this.

No I don't eat dragon, cause, uh, it's not a meal for peasants, it's a meal for kings, and I'm sort of a common man. But they don't eat us, it's a common misconception. They actually eat gold and treasure -- that's why they're always sitting on a pile of it.

Charlie