Rei first appeared in Triankha Hostipal talking to an old woman, who was later known as Onba. They talked about Rei quiting as Onba's accomplice. Onba realized that Rei was working with someone. But before Onba finished speaking the name of Rei's new accomplice, Rei tried to kill Onba with a fire magic but misses and Onba escaped.
The fire magic instead hits Tatsuya and the others. As Rei passed through thinking that they were dead, the group stood up and stopped Rei. Sogreth accused Rei as the monster loosed in the hospital. Rei said that he wasn't the monster but was the one who released it. Because of this, the group draws their weapons to fight Rei. He then saw the gemstone on Tatsuya's sword and claims that he was "the one". Rei decided to let them go and pointed out where the monster was, saying that it was tearing apart a group of people. Sogreth exclaims that it was Kyte's team and quickly left. Yuno followed and warned Rei that she never forgets a face. Rei left saying "We'll see if the voice below calls to you the way is did to me."
Rei showed again at the real world in Miki's hospital room. He came to give Tatsuya a little lesson. Rei told him not to trust anyone. As the bells rings, he warned Tatsuya to not get in his way or he'll kill Miki, Yuno and Sogreth. Rei left afterwards.
Rei reappeared at the Paulta Mines and tried to kill Tatsuya and his friends. Volgh, the Firewyrm instead took the hit to protect them. Rei took the gemstone in the Firewyrm's eye. Rei left after summoning a subordinate to take care of Tatsuya and his friends. After gaining his fourth gemstone, Rei returned to Miki's hospital room to kill her but hesitates. Tatsuya appeared and fights Rei. In the middle of their battle, the bells ring and Rei flees.
His last appearance is in triankha Hospital basement.When the party transported to the dark realm.he summoned undead Volgh to fight the party alongside him.After defeated for first time he removed the gem of darkness from the mirror,transforming him into grand wizard,boosting his power even more,but Tatsuya manages to defeat him second time.
Because the gemstone was removed,the seal was broken.Demonkin start pouring out to the vision.being critically injured,the demon king revealed that he double crossed him.Tatsuya is given choice to teleport away or help Rei.Should Tatsuya choose to help Rei,They manages to hold out the demon for a brief moment.Then,Rei reveal his dark past and regret,before teleporting The party back to swamp of grief.