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

Rydell is an old running buddy of Crab's. Years before, Rydell came to Crab with the idea of robbing an armored truck. However, Crab has a toothache and cannot take part in the robbery. Crab does not know what happened during the robbery, but suspects that Rydell took part in it with Frank and that Frank killed the guard who died. Crab goes to jail for this crime and comes to Arkansas with the hope that Rydell will tell Jimmy the truth about that day.