Mohammed is a minor character introduced in GTA IV.
Mohammed is a Middle Eastern taxi driver, apparently in his late thirties, employed under Roman Bellic; Roman is shouting to him over the radio in an early cutscene, but he first appears as the one who drives Niko Bellic around when Niko receives free taxi rides (Roman's friendship ability). Early in the game, he picks Niko up in one of Roman's Esperanto-based taxis, but the fleet is upgraded to use Cavalcades right after Roman buys a new safehouse in Algonquin -- a phone call from Roman suggests that he had a sizeable gambling win around the same time. Mohammed is very short tempered and impatient, although he is relatively civil to Niko at first; this quickly changes to open hostility, as he apparently resents not only Roman for his company's perceived low wages, but also Niko ("dog lover", in Mohammed's parlance), who he believes is a lazy freeloader for getting free rides (Mohammed claims he is not paid for these trips).
Mohammed claims that he only remains a taxi driver so that he can seduce the female tourists he picks up from the airport; he says he loves his wife, although evidently he feels no need to remain physically faithful. He suggests that he has six children -- his wife has had none, but he estimates that with all his adulterous activity, he has probably had around that number. The truthfulness of his boasting is left to the player's own conclusions.
The player can kill Mohammed any time, but he'll always come back.