"I'm always proud to fight with you, sir!"
-Captain rex of the 501st legion-
In the fusion war, rex can be found at pokey oaks south when not on the battlefield. He gives the players helpful tips and his dual blasters as a reward for the mission: Doppelganger rex.
Later in the war, ben suspects that rex is playing both sides of the war. Ben attacks rex, but rex gets help from gwen tennyson, and ben is defeated. Ben apologized, as he thought he saw rex kill bloo and mac. Rex said it was not him. Later, the player faces and defeats fusion captain rex. It is revealed that bloo was killed, but mac was taken to fuse's dungeon.