Jimmy is an African-American citizen in Liberty City. Jimmy took a cash loan from the Faustin Bratva's loan shark, Vladimir Glebov, and did not pay his debts in time. Niko Bellic is sent to steal Jimmy's car as interest of the loan and take it to a garage owned by Vlad.
Jimmy and his associate are likely M.O.B. members.
There are three results of Niko's encounter with Jimmy.
If Niko kills him, in his phone call to Vlad, he will say "He was reluctant. Now he is dead", and Vlad will respond "You stupid bastard. At least I got my money".
If Niko spares him but knocks him out, Niko will say "He's a little bruised, but he gave it up easy enough", and Vlad will respond "Good. Dead guys can't pay interest. You're learning".
If Niko ignores Jimmy altogether and just takes the car, Niko will say "It was easy. I didn't even have to touch the guy", and Vlad will have the same response as if Niko knocked him out.