If Urdnot Wrex does not die, then he will appear in both Mass Effect 2 and Mass Effect 3 as the leader of the Clan Urdnot.
Please notice that you will have the opportunity to kill him at Virmire when Captain Kirrahe emphatically states that Saren has found a cure for the genophage, and it needs to be destroyed. Therefore, in Mass Effect 2 Shepard and Garrus will be vaguely remembered from their last meeting at the Citadel Tower.
By making this decision, Garrus will be familiar with Shepard when they reunite during Mass Effect 2.
