Their relationship initially began as hostile on their first encounter in Volume 5 when Kuroka tries to kidnap Koneko and as a member of Khaos Brigade. Kuroka attacks Issei and overwhelmed him, taunting Issei that he's much weaker than Vali. When Issei attained his Balance Breaker, however, he became powerful enough to withstand her attacks, much to her surprise. While he stops his attack just before hitting her, planning to show her mercy, he declares that he will not stop next time if she tries to take Koneko.
She reappears in Volume 7, claiming Issei became more manly and asks him to have children with her because she is interested in strong genes and the holders of the Sacred Gears of the Heavenly Dragons would potentially make powerful offspring, but Koneko refuses to give Issei to her. The reason why she proposed Issei her offer was because she was initially refused by Vali. In Volume 11, when Koneko enters mating season, Kuroka attempts to seduce Issei after she managed to revert Koneko back to normal. Although this attempt is interrupted by an exhausted Koneko, declaring that Issei is hers. Kuroka leaves and quietly tells Issei to look after her sister, showing how much she cares for her sister and how much she trusts him. While recovering due to her injuries from Cao Cao, she explains to Issei the reason why she killed her master. Issei later lessens his hostility towards her, claiming she's clumsier than he is. Unamused by the conversation, she claims that she doesn't want to hear that from him, but she thanked him as he leaves, though she pretends to be asleep.
In Volume 14, during her stay at the Hyoudou Residence, they became friendlier as Issei was willing to help the two sisters reconcile their relationship when the time comes, adding that Koneko's smile was the most important thing to him. Kuroka finally understood the reason why all the girls around Issei were falling for him, claiming that he was more charming than normal handsome men, indicating that she has developed romantic feelings for Issei as well.
In Volume 15, she became slightly closer to Issei, hugging him while rubbing her cheeks against his, telling him that he shouldn't become like the Evil Dragons and remain being the Oppai Dragon, while sitting on his lap. It is also revealed by Issei that ever since she has been staying with him, she has been getting closer to him more often.
In Volume 24, their relationship becomes even closer than ever upon learning the full truth about the Nekomata sisters upbringing, they had a heartfelt moment where Kuroka admitted to him she kept the hard truth hidden from her sister to protect her. Kuroka then hugged Issei pleading to continue living peacefully with him and her sister, promising to make amends for any trouble she's caused them and fight off any threats in the future, Issei hugged her back assuring that he will protect her and Koneko, thus establishing each other as family now. Through their communication, Kuroka revealed that she fell in love with Issei because he was very kind to both her and her sister Koneko and gave them a place to live and seeing him smile made her heart filled with love. Kuroka then confess her love for Issei which he reciprocates and wishes to have children with him. After the confession, Kuroka begins to call Issei by his name and kisses him for the first time.
Kuroka has been known to tease Rias with the nicknames that Bikou made up for her.
She cares a great deal for her younger sister even though Koneko really hates her for what she did to their former master. Kuroka felt somewhat depressed and distant towards her sister for what she had done, even knowingly impossible for her to be forgiven. During their reunion at the Young Devils Gathering, she plans on taking Koneko by force, even engulfing her in a fog of poison. She later admitted to Issei that she was happy at their short reunion.
She even put herself in harm's way when she was attacked by a reflected attack from Cao Cao as she wants nothing to happen to her younger sister and wants to protect her from everybody. She has put her trust in Issei to protect her since she has the potential to become a powerful and special member of her tribe. During the assault of the Grim Reapers, she jokingly asks Koneko if she want learn more about senjutsu and youjutsu from her, of which she later determinedly accepts.
Their relationship is somewhat improved as Issei stated that Koneko became a little happy during their training. She also planned on teaching her as an older sister efficiently, even reading extensive books regarding the basics of the energy flows within the body, senjutsu, touki, and such despite being an expert. Koneko became strict with Kuroka, telling her the only reason she could stay in the Hyoudou Residence was so that she could train Koneko and Gasper.
Kuroka tried to have Vali make love to her so she could have strong children but he refused her. Vali cares for her as a comrade as he was enraged when she was injured, especially when the injury was caused by his own technique (being reflected by Cao Cao). Kuroka also shows that she cares for Vali and respects him to a certain extent calling him "leader".
Kuroka and Le Fay get along well being partners and are often shown working together. She even recommended Le Fay to be Issei's pact magician.
Bikou and Kuroka constantly bicker with each other over the smallest issue when they are in some sort of troubled situation. Despite so, the two of them get along well because of their similar personalities and stupidity.
As a child, Kuroka dearly loved her mother, and was saddened when she left her and Koneko behind to support their negelctive father in his research though Kuroka understand that Fujimai was trying to get her lover to acknowledge her and her sister but to no avail. Fujimai died in an experimental failure in her father's research to artificially create Super Devils, leaving her and Koneko orphaned.
Despite having almost no interaction with her father, Kuroka clearly understands her father's personality and despised him for being completely neglectful towards both her mother, Koneko and herself due to his single minded devotion to his research in creating artificial Super Devils. Due to this, Kuroka never mentioned about her parents to Koneko as she does not wish her sister to know the cruel truth about their father.