Yuko Nishikiyama (錦山 優子, Nishikiyama Yūko) is the late younger sister of Akira Nishikiyama.
Yuko was raised at Shintaro Kazama's Sunflower Orphanage, alongside her brother. She, like her brother, was adoptive siblings with Kazuma Kiryu and Yumi Sawamura. However, she became sickly at a young age and spent much of her time in the hospital. Nishikiyama's character profile in Yakuza 0 (set in 1988) mentions that he has a soft side in caring for and supporting his sickly sister.
Like a Dragon: Prologue
Yuko is first introduced as the one to motivate Yumi to open herself up to the rest of the orphans by getting her to play with them. This causes the four orphans to become closer together. At some point, she suffered from an unnamed illness involving her heart, which caused her to become hospitalized. The resulting medical bills for hospitalization and indefinite stay were paid by Kazama and the Dojima Family. Though Nishiki was grateful for their help, this also caused him to feel indebted to the Yakuza, joining them in hopes of continuing to support Yuko's recovery whilst living a lifestyle that would suit him.
Yakuza/Yakuza Kiwami (1995-2005)
In 1995, Nishikiyama shoots Sohei Dojima in response to him attempting to sexually assault Yumi. Because Yuko is heavily reliant on her older brother and her final surgery is coming up, Kiryu decides to take the fall for Sohei's murder in the hope that it will allow Nishikiyama to still spend time with her.
In Yakuza Kiwami, exclusive flashback cutscenes flesh out what happened to Yuko. The final surgery is a failure, but Doctor Kiminobu Hiyoshi tells Nishiki that he can try getting a new heart on the black market, but that it will cost him 30 million yen. In what costs Nishikiyama a lot of strife and humiliation, Nishikiyama enlists the help of his abusive and disrespectful lead subordinate Matsushige to get the money. Eventually he gets it all, but once he pays it, Hiyoshi turns out to be a fraud who used the money to pay off his debts before disappearing.
Shortly thereafter, Yuko dies in 1996. This, combined with the fact that Yumi's disappeared after Sohei's death, Nishikiyama's own feelings of inferiority, and the constant damning comparisons made of him to Kiryu by those around him, cause him to descend into darkness. As he sits in front of his shrine to Yuko in his office, he grabs a tanto knife and holds it towards his own stomach about to commit seppuku, before Matsushige interrupts. Nishikiyama finally snaps when Matsushige continues to shame and criticize him, and stabs him to death, thus completing his transformation into the man Kiryu met after he left prison.