Kuroda Yuriko

Yuriko was the only child to boss Ryuichiro Kuroda and his wife Kyoko. She is also Kumiko's deceased mother. She grew to hate the life of a Yakuza family member and eventually ran away to avoid marrying a mobster like her mother. She's mentioned several times in the manga and there are photos of her along with her husband.

Background
Very little is known about Yuriko's life and childhood. Its assumed that at some point, she realized her father was a Yakuza boss and for a time had accepted this as a fact. But over time, Yuriko saw the many hardships her mother faced as a Yakuza boss's wife and she came to hate Yakuza lifestyle. Eventually, she left her home and settled down away where no one would know her family connections.

She eventually married a man named Yamaguchi and the two had Kumiko. They raised her as normally as they could. But at the age of 7, Kumiko would lose both her parents.

Kumiko's Real Father?
In the manga, a secret Yuriko kept is revealed. Like her mother, Yuriko developed feelings for a man in the Yakuza. A young man named Kousuke, who had been working under her father. Though many didn't know, Kousuke returned the feelings and the two developed a relationship. Though after a big fight, it was the last straw for Yuriko.

After seeing Kousuke badly hurt from a fight and remembering what her mother's hardships as a Yakuza wife; it drove Yuriko to leave. Kousuke did at first try to persuade her to not to leave but to no avail. After a moment of silence between the two, Yuriko offered Kousuke to come with her, hoping the two could both leave and live a quiet life. To her disappointment, Kousuke refused, not wanting to give up his lifestyle.

Kumiko eventually heard of how he and her mother were once lovers. And ponder for a long while if Kousuke was her father, but eventually dropped it.

Kyo asked Kousuke if he was Kumiko's real father. To his surprise, Kousuke only answered that Yuriko didin't tell him she was pregnant when she left; hinting they had been intimate. But in all honesty, he's not very sure. Even if he was, he understood why Yuriko didn't tell him. Yuriko didn't want her child to be born to the Yakuza.