Frank Grimes, Jr.

Frank Grimes, Jr. is the son of Frank Grimes and an enemy of Homer Simpson who wants revenge for his father's death.

Biography
Homer Simpson is subject to several murder attempts. Sideshow Bob is roped in to help him find the perpetrator, who is revealed to be Junior, Homer's mechanic. No one knew of his true lineage until he revealed it to them upon being cornered: he is an illegitimate child; his dad "happened to like hookers," and one of them gave birth to him.

Homer "killed" his father by driving him insane, so Junior was trying to avenge his death. He even "killed" Duffman. He also becomes more infuriated in each passing moment he shares with Homer (just like his father). He has yet to reappear in future episodes.

Non Canon Appearances
In Simpsons Comic #125 Homer was framed by Mr. Burns for dealing nuclear weapons and arrested. He was cellmates with Sideshow Bob and they escaped from jail, by going through a tunnel in a cell occupied by a masked prisoner. The three fell into Mr. Burns' secret lair, where the mysterious prisoner was revealed to be Grimes. Sideshow Bob then fell on him and both were knocked unconscious. Homer was cleared of his charges, whilst Bob and Grimes were presumably sent back to prison.