A trapped UN sailor who guides Holden through the plague-ship, then — secretly infected — sacrifices himself to trigger the self-destruct.
Larson is a compact Earther sailor with pale skin and close-cropped flaming-red hair, aboard the UNN Agatha King when a protomolecule monster boarded and the ship sealed into lockdown. Trapped in an airlock storage room, he watched the horror on the security feeds until they went dark, then hid in a locker. When Holden enters in the bulky red hazmat suit, Larson mistakes him for a monster and attacks with a wrench, then — believing Holden an admiral — eagerly guides him toward the CIC, working out from location checks that the whole crew has been herded into the forward galley (now full of zombies).
He leads Holden through the swarm, gets his suit sleeve torn and stained brown in an elevator door, and reaches the CIC. When they find the self-destruct is a no-timer trap that would strand whoever presses it, Larson reveals his infection and volunteers: he watched the Eros feeds and won't risk becoming 'one of those.' He sends Holden to a higher airlock, pulls off his helmet, takes a long breath, and presses the plain white button — turning himself and the ship into a brief star. Holden vows Admiral Souther and everyone will know what he did.
A compact Earther with pale skin and flaming red hair cut close to the scalp, in a UN naval uniform then a stolen environment suit with a small brown-stained tear at the sleeve.
Trapped UN sailor who guides Holden to the codes and sacrifices himself to detonate the plague-ship.