Jonnie S was scheduled to be a character in the Grand Theft Auto series who was dropped during the development of Grand Theft Auto Advance. Jonnie S was scheduled to be a guy who owes money to 8-Ball. According to some lines of cut dialogues of the game, he was supposed to appear in the cut mission called "Runaway Idaho".
It is more likely that this mission is a early version of the mission Show The Money given by Jonnie, the bartender. So it's possible that "Jonnie S" its a beta name for Freddy, a character who owes money to Jonnie in this mission.