Keisuke "Kei" Mark is the Blue Raiju Nefilim with an elemental connection to water. He's an idealistic young man who's quiet with only a few words, disciplined but anxious to a certain degree. Nevertheless, he cares for others’ well-being, even if he infrequently shows that side of himself. Two years before the event of the story, there was a heated confrontation between him and his parents, which only led them to disappear and be presumed dead during their trip to Japan, leaving Kei with devastating guilt for not making amends with them. After learning about his power and his lineage, he was convinced by his brother, Jinsuke, to join the team to stop the Kemuri Clan, as they have a connection to their parents' death. Although Kei is neither an experienced fighter nor has any experience living in another country, he's willing to learn and adapt to his new surroundings.