[ Explanation time, then... Kazu wasn't around when this happened, but he knows the story. ]

Chase was actually in the first official battle of the war. He was an interceptor pilot who sacrificed himself to temporarily disable the smoke and buy everyone else time to retreat when the city fell. The damage he took was beyond anything they could regenerate, some problem with how being partially destroyed by nanotech affected him, so the only way they could keep him alive was on life support in a tank. For the past four years he was just a torso, head, and one arm. Losing his body still changed a few things, but in terms of interaction with the world things aren't really different.