Guri[]
He first encountered her in his house and she told him that she accidentally wrote his name down on a Kiss Note, stating that if Seiji himself, don't kiss any girl she chose for over 24 hours, he will remain a virgin forever and Guri will die shortly thereafter. Though she is one of two characters (hence, Seiji and Guri herself) being stabbed by Akane for several times throughout the story, they were immune to death, meaning that both Akane and Seiji had granted an Angel's Blessing when the two names were in the Kiss Note, which Seiji was immune from death but unlike Guri's immunity from stab wounds, physical pain and death, Seiji and Akane can still feel physical pain. Since the start of the series, Guri wasn't supposed to be with him but after an unlikely event such as when Seiji kisses Guri because he misunderstands the process about his virginity and her death, this triggers Akane's yandere personality as Akane thought that Guri was attached to Seiji.
At the end of the anime, she acknowledges that she has genuine feelings for him and kisses him, to his surprise. Guri confesses that she wants him to love her and Seiji was left blushing by this declaration.
Akane Hiyama[]
A woman who seems to be madly in love with Seiji and a love rival against Guri for Seiji. When he first encountered and met Akane at school with Guri, the latter said about an unlikely event at Seiji's house which reveals her true colors such as her Yandere personality, which scares Seiji to the point that if he wouldn't be ready to face her. She is a "yanderistic" character which is shown when another woman tries to be near or have a relationship with Seiji. In "Episode 5", she would even go as far as trying to commit suicide with a knife, should Seiji have died though he was actually sent to Heaven after he was knocked out, temporarily. Prior to the start of "Episode 1" of the Renai Boukun series, Seiji had previously met Akane before. When his eyes were blinded as he was scratched by his pet cat, Akane helped Seiji with both the cat and as his guide. Afterwards, Seiji told her that she is very kind and he had stronger feelings for her.
She officially becomes his fiance at the end of the manga, along with Guri and the other girls.
Akua Aino[]
Seiji's younger sister. During her childhood, she is far looking girly than her present age, Prior to the start of the series throughout the early years before Seiji met Akane, she became more tomboyish(despite she was a feminine) when she was getting older, simply to attract her brother's attention. She began to change herself however when Seiji began spending less and less time with her in order to get his attention back. Despite this, Seiji failed to notice her to the point where it developed into animosity against Seiji, much to his confusion. When Seiji had stronger feelings for Akane and he is in newfound harem, her discontent grows further, to the point that she tends to injure Seiji whether if he didn't understand about her emotions, marking her as a 'tsundere', meaning she loved him unconditionally but at the same time, hates him due to her animosity. No one knows if Akua wanted to have a close relationship with her brother or if she had stronger and romantic feelings for Seiji similar to Akane's.
Yuzu Kichougasaki[]
Seiji was surprised to hear Yuzu was Akane's half-sister. At first, Yuzu saw him as a rival and obstacle for Akane's love, even believing that Akane's love for Seiji is not real due to the effects of the Kiss Note. She even attempted to find ways to blackmail him or trick him into a poorly executed trap in order to make Akane dislike him, only for her to fail and get scolded by her sister.
Despite their turbulent standing initially, Seiji shows his concern for Yuzu as well as expressing how strong and driven she can be, their relationship begins to improve as she starts developing feelings for him. Yuzu becomes something a tsundere towards Seiji as she becomes more concerned for his sake as well as wondering if they should kiss, all the while denying having any feelings for him. On numerous occasions, Yuzu would share a kiss with Seiji, albeit accidentally and sometimes intentionally. She also blushes whenever she is close to him and when he shows care and consideration for her well-being; such as taking care of her injury from a cut, bringing an umbrella for her during a rainy day and carrying her like a princess when fleeing from the demons in Hell.
Yuzu seems to see Seiji as a good friend, as she soon began to fall in love with him and even becomes one of his brides alongside Guri and Akane.
Shikimi Shiramine[]
-These are the ones I know but I can't seem to write the rest of these main or supportive characters, especially the information about their relationship.