Who is Young Frank from Somewhere in the Darkness and what is their importance?

Frank is Mavis' son. Frank is a little older than Jimmy and is fixated on boxing. Frank appears to want to impress Crab when they watch him box. However, when Jimmy returns to speak to Frank alone, Frank is rude to him and threatens to beat him up. Jimmy proves his bravery by facing off with Frank, but later wonders what might have happened in a fight between them. Crab seems to think Jimmy could not have won but encourages him to stand up to him just the same. This confuses Jimmy, leaving him wondering if Crab wants him to get hurt.