Kuro no Shoukanshi

Kelvin begins a new life in another world having traded away memories of his previous one for exceptional abilities and the rare S-class Summoner skill. He also apparently fell in love with the goddess Melfina before reincarnating and chose her as his first contracted companion, although summoning her physical form will require far more magical power than he currently possesses.

That gives Kelvin a convenient excuse to do what he quickly discovers he enjoys far too much: fight strong opponents. He takes adventuring jobs, forms contracts with unusual companions, and keeps seeking harder battles while hiding just how extraordinary his abilities are. The quiet newcomer he pretends to be is soon surrounded by a growing party that realizes their summoner is less interested in fame than in the thrill of seeing what happens when the next enemy is actually stronger.