Mallow was a Nomai traveler from the Brittle Hollow crash. Their spouse is Avens and they are "enthusiastic".
They believe that the Eye of the Universe is in a distant orbit around the Sun and they suggest building a probe to try to find the Eye visually because they already know what it looks like from the Quantum Moon. Mallow adjusts the power setting of the cannon after talking with Avens about the setting most likely already being set low by their friends assuming they were going to raise it anyway.
Mallow's remains are presumably with Avens' on the Construction Yard where Avens worked, to meet him there after the failure of the Ash Twin Project.