Thor is a genie of the ring. His brother is the genie of the lamp. He is a very low-level demon and his magic only lasts for 5 minutes.
Originally, the ring belonged to Prince Russet's mother, who gave it to Russet later. Lys grabbed the ring to keep it from falling out a window, and rubbed the ring, so she became Thor's master instead. When Lys and Siva leave Russet's palace, Russet sends Thor along to keep an eye on Lys and "protect her from demons".