Kazuki's mother served Toji Jounouchi as a maid. They became intimate, despite Toji being married and having children already. When Toji's wife found out about her husband's infidelity, she became mad with jealousy and Kazuki and his mother had to run away. When Kazuki was six years old, his mother became sick and passed away from her illness. Before she died, she informed Kazuki about his patronage. He came to hate his father and plot to get revenge against him.
Kazuki is very obsessed with Hazuki. When she jokingly states that she wasn't dating Ryo, he becomes jealous and angry at the concept. He states that if that were true, he would make him "disappear". When she learns about his history and fully accepts him for who he is, he clings to her and asks her to move in with him. In his "Extreme Love End", he assumes that the Jounouchi family is preventing Hazuki from being with him and kills all of them.