It's Nick Reinhart from Tera Melos. Someone posted pictures on r/deathgrips of the entire booklet. The third dude (Andy aka Flatlander) handles production and engineering. Guitars were mostly Nick, and then some guy named Julian helped with extra guitar work and the organs/keys.
Jenny Death is real tight. Makes me want to BREAK MIRRORS WITH MY FACE IN THE UNITED STATES.
Also, looking at the personnel for the entire The Powers That B, it looks like Zach Hill basically played the entirety of niggas on the moon on a virtual drumkit. This includes Bjork's vocal samples which is fucking crazy.