Marvin

Marvin is a M.A.D. Monk found at the Canyon of Titan.

Details
He is assisting Sarah and will sacrifice himself to kill Klink - inadvertently blowing Sarah's legs off and hurting her more than the raider ever did.

It's possible to kill Klink and protect Sarah and Marvin from death, but it requires approaching him from the other direction and shooting him with a sniper at maximum range.