Stevie is a black-market vehicle trader in Grand Theft Auto IV. Whilst not encountered in person, he contacts Niko Bellic via text message with stolen car delivery requests. He has a garage called S&M Auto Sales located in Bohan, where the player delivers stolen cars in exchange for cash. Only the requested vehicles are accepted until all thirty car deliveries are complete; at that point, any car may be sold for cash. The reward is dependent on the model of the car, and its condition.
Brucie Kibbutz and Stevie are apparently friends and have had business dealings in the past, as it is Brucie that asks to borrow Stevie's gold Comet to race in the mission, "No. 1." Upon winning the race with Niko at the wheel, Brucie is so impressed that he turns the Comet over to Niko (implying that the car is not legitimately owned by Stevie, but is simply a "company car" obtained through his stolen car business). This incident is referenced twice more in the game: first, when Brucie calls Niko to inform him of Stevie's interest in employing him to steal cars, Niko asks if Stevie wants his car back, to which Brucie replies no; second, Stevie eventually states in a text message that he misses his Comet, and asks Niko to obtain another one that he has had his eye on: a plum-colored one parked in Westminster.
Stevie makes a possible on-screen appearance at the end of the mission I'll Take Her..., albeit briefly. When the player takes Gracie to the safehouse, Gordon says "There he is! Stevie, dump the car." Then a dark-haired man in a tan sportcoat and grey slacks gets into Gracie's Feltzer and drives off. Whether this is the same "Stevie" Niko usually deals with is unknown -- Stevie the car-trader is not known to have any connection with the Irish unless Stevie wanted the car to pocket the cash and nothing more. After the mission, Stevie is seen driving off, the player can kill him and take the car, though this does not affect anything in the game and it is not canon, as he is not meant to be killed.
A map of the locations of the Car Thefts can be found on whattheydonotwantyoutoknow.com posted by Stevie.