Tattoo was a thrall working for the Jotunn, Baugi.
Tattoo is amoung the thralls guarding Kvasir's Mead in Gunlod’s Cave. When Magnus Chase, Samirah al-Abbas, and Mallory Keen arrive, the daughter of Frigg temps him and the other thralls with the Whetstone and they inadvertently maim themselves when she tosses it into the air.
Like his fellow thralls, Tattoo had an iron collar around his neck. He wore nothing but a loincloth and a whole lot of muscles. His bronze skin, shaggy hair, and beard was dripping with perspiration. His face was impressively tattooed.