Before he became the boss of the infamous Wada group, Kousuke worked under boss Ryuichiro Kuroda. Aside from working in the Kuroda family, he was very good friends with Kyou, he considered him a good friend. To date, the two still have a friendship to a point he's willing to confide things to Kyou,
Before becoming head of the Wada group, Kousuke worked for the Kuroda family. Only a few people actually knew he and Ryuichiro's daughter Yuriko were having a secret relationship. Those that knew the couple, hint they were had strong feelings for each other and that Kousuke may still love her.
However, the romance wouldn't last long. Unlike many women married into the yakuza life, Yuriko had no desire to enter this world. She didn't want to be the yakuza wife and pretend she didn't know what her husband was doing. Or pretend the risks he was taking on his life, and she eventually vowed to leave her father and the Kuroda family's life behind.
Its then Kousuke hears this and begs her not to leave. But Yuriko will not change her mind and seeing him hurt from a fight, only proved her point why she didn't want that life. She did offer Kousuke a life with her, if he left the yakuza. But he refused, admitting he didn't know any other way of life and that he'd feel useless trying to look for a "normal" job. Yuriko left and the two never saw or spoke to one another, even up till her death. It is around the time she left, she eventually gave birth to Kumiko with her new husband.
After her death, Kumiko was given to the care of her grandfather. During her youth, she remembers meeting Kousuke. But to her recollection he was very shy and seemed nervous to interact with her. Yet, even during his shyness, he seemed very kind to her and tried to befriend the little girl.
A few years later, its revealed to Kumiko of her mother's relationship with Kousuke. This makes her begin to wonder if he could be her father, believing they had similarities in appearance. She even protects him from an attempted assassination. But Ryuichiro, is extremely angry that she put her life at risk and is apparently trying to dig up the past.
But both sides decide to leave the matter alone. Its when he's alone with Kyou, that is revealed the two were intimate, but due to Kumiko's life as a teacher, he believes its best she have a the man Yuriko married as her father than him. But he admits there are moments where he'd like to think she's his daughter.
Upon hearing this, Kumiko admits while she loved both her mother and father, a part of her wants to imagine Kousuke as her father as well.
Kousuke never appeared in the anime verison Gokusen.
Since DNA tests were never taken and there hints of intimacy between him and Yuriko, there still is a possibility that he could be Kumiko's father.