Serin: Their relationship is one of the most complicated pieces in both series. Jae-Hyuk is uncertain of his feelings for her. He is divided between mistrust and desire. At first, he doesn't understand the things Serin does-creation of him into an undead, bringing Jae-Hoon into Chaos etc. Overtime, he begins to notice how she is becoming equally important as Jae-Hoon. He begins to realize he loves her when he attempts teleporting after her despite unknowing what risks are involved. While the two break all communication at the end of Immortal Regis and he angers whenever her name is mentioned, she still lingers within his mind despite his self-proclaimed hatred of her. After learning her continuation of sending even after breaking communication, he forgets his anger momentarily out of the wish to see her and ask why did things turn out this way. Jae-Hyuk's refusal of her dying is a major plot piece in both series, he will shield her without a care for his well-being and refuses to listen to prophecies preaching her death, he is shown telling Hekalroth he will change history to make sure her death doesn't happen.
While he imprisons Serin and refuses to allow anyone to come into contact with her after her betrayal, others question if he really wishes for her to suffer or if he couldn't find it within himself to kill her. (It is noted that his harsh treatment of Xix changes slowly after he questions if the boy is their child).
Jae-Hoon: Jae-Hyuk's main reason for existing he believes is to make Jae-Hoon happy after the lost of their parents. His whole journey in Immortal Regis is returning to home to Jae-Hoon; while In Cavalier of the Abyss he wishes to both save and destroy his brother who is now in Caladborg's control. Jae-Hoon is a light in Jae-Hyuk's desperate life of working multiple jobs to provide for the boy; he wishes for nothing more than seeing the child smile without pain. Jae-Hoon is the most important person in Jae-Hyuk's life until Serin becomes of equal performance. Jae-Hoon is one of the main reasons Jae-Hyuk and Serin have complications in their relationship. Jae-Hyuk wishes to love Serin, but his loyalty to his brother causes him to hold these feelings back.
Depore: Jae-Hyuk considers Depore one of his closest allies and one of the few people he continues to confide in after turning into Regis. Depore has romantic feelings towards him, which Jae-Hyuk is aware, but he lets her know she is important to him but not in the sense of romance.