Melfina

The Goddess of Reincarnation, she is the one responsible for giving Kelvin his powers in exchange for his memories. As she is a godess, she has a very high level, over 100. She is so strong that Kevin himself, an S-rank summoner, couldn't summon her when he entered the world. Her true form was hidden, appearing as an interactive game menu until creating an artificial body for herself which Kelvin can summon. Spoiler