Can Defenders die?
From personal experience, I can confirm that when a Defender loses all their life, they collapse where they're standing and start calling out for help.
Players can then hold the interact button (X on Xbox, and □on PS4) to revive the defender and get them back in the fight.
I personally have never waited very long to revive one, but I did not see a visible timer, so I assume they remain revivable for the rest of the level.