After a fight, something bad happens to Character A.
This could be one (or more) of these or anything you could think of:
They lose their T-Cog and it can't get replaced for some reason
They fall into a coma
They lose a limb and it can't get replaced for some reason
They optics are damaged so they can no longer see
They lose their wings
Their audial sensors are damaged so they can't hear anything