Find a novel that travels through different worlds

The protagonist is a seductive type of person who seems to be in love The book starts in the modern world. The main characters seem to be students (pretending to be), and there's another one who seems to be a modern mercenary type. He and a brother who risked their lives. I forgot how they traveled. The three of them traveled to a different world. In that world, people's hair was colorful, but there was no black.(The one with black hair seems to be the type of Savior.) The main character has black hair. The two people who wore it with him had black hair, but after they wore it, their hair changed color. The protagonist was worshipped like a savior and fell in love with the mercenary. Later, a god came out and said that the mercenary's brother had black hair. It was the god who sent him to another world (the world before they came) to train. When the time came, she would wear it back, and the protagonist and his man were an unexpected accident. I don't know who they are. After that, I forgot, as if it hadn't ended (two or three years ago) Ask for the title and author Additional points,