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.

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

Dee: You've been really stressed so I thought I'd take you to a spa day. Just the two of us.
Charlie: A what day?
Dee: Spa day
Charlie: What is this word, spa? I feel like you're starting to a say a word and you're not finishing it. Spaghetti? Are you taking me to a spaghetti day?

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