"Red" was a thrall working for the Jotunn, Baugi.
Red is one of the thralls standing guard over Gunlod’s Cave when Magnus Chase, Samirah al-Abbas, and Mallory Keen arrive for Kvasir's Mead. He and the other thralls accidentally kill themselves when the daughter of Frigg offers them the Whetstone.
Like his fellow thralls, Red had an iron collar around his neck. He wore nothing but a loincloth and a whole lot of muscles. His bronze skin, shaggy red hair, and beard was dripping with perspiration.