Hajime was a complete otaku and always came to class with a sleepy expression. He was always bullied and was disliked by both of his male and female classmates for being close to their idol-like classmate Kaori. After wishing for his annoying classmates to be summoned to another world, the next thing he saw was they were on top of a pillar like structure in another world to become heroes.

He obtained the job of "Synergist" and continued to be bullied due to also being weak in this new world. He eventually fell into an abyss due to being hit by a classmate's fireball where he survived though a hell-like environment to become the powerful man he is today.

Hajime at the beginning of the story is said to have black hair and also has a slim body. He also had a kind face and attitude. But after falling in the abyss and eating a wolf-like monster, his body began to violently degenerate. When he drunk the sacred water, it made his body heal by destroying his bones and muscles then healing over and over again until he got a very muscular body due to overcompensation.

Due to the stress of breaking and healing, his hair turned white, a victim of the Marie Antoinette syndrome. His eyes also turned red and his body gained tattoo-like red lines which is the sign of him becoming a demonic monster similar to the monsters he ate that also possessed red lines along their bodies.

His personality was at first kind though he was considered incompetent. But after falling in the abyss, he became cruel and would kill any person who wants to cause him misfortune. He would also smile in front of danger which made Shia call him a new species of monster called "Predator." He may be cruel but he cares for his comrades.

(Source: Arifureta Wiki)