Touka was one of the students summoned by the Goddess Vicius to become a hero in another world, but he was abandoned and sent to his death by Goddess Vicius.

After surviving and escaping the Ruins of Disposal, Touka went on an avenging quest to kill the Goddess Vicius. On the outside, he appears to be a kind, honest, good-natured and straightforward man. But because of his childhood, full of dark memories of being tortured by his own parents, he has developed a hidden personality. His guiding principle is that of an eye for an eye. Later, when his parents left him with his aunt and uncle, who became his foster parents and the people who really cared for Touka, he suppressed his dark side and became kind, honest and happy.

Later, when he was betrayed by the Goddess Vicius, his dark personality took over again.

(Source: Wiki)