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.
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.
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.