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