Henry 1.0 is Leviathan's long lost pet snake. It mysteriously went missing as a snakelet and was later found wandering in the underground tunnels of the Demon Lord's Castle as a giant serpent. Its first appearance was in Lesson 7-17.
MC, Mammon, Leviathan, Beelzebub, Asmodeus, and Solomon were sucked into a portrait of one of Asmodeus's ex-lovers in the Demon Lord's Castle and teleported to the underground labyrinth, where they came across Henry 1.0. Although Leviathan recognized Henry 1.0 on sight, the snake seemed to have no recollection of its previous owner, and tried to attack them. The group was able to escape when Solomon amplifies Asmodeus's powers to charm Henry 1.0.