First appearance: Chapter 242

Part of the Osaka Gantz Team.

She first sees Kato Masaru help a family from one of the aliens. Bewildered by his actions, she asks him what he was doing. When he responded that he was saving the family, she immediately laughs at him. Initially she follows him around while waiting for him to show a contradiction in his actions and words.

When he does not show one and even puts himself in danger to save others, she leaves his side. Feeling strangely guilty, she returns to help Kato defeat the alien he was facing. Unfortunately she gets in his way more than anything else and Kato ends up rescuing her. Yamasaki continues to follow Kato throughout the next few chapters, and finds herself attracted to his kindness and compassion. In the end she tells him about her son (she feels closer to Kato after he tells her of a similar situation between himself and his younger brother).

After Kato announces that he will act as bait for the 100 point alien, Yamasaki asks Kato if he and his brother will move in with her and her son. In order to make this reality, she vows to not let Kato die no matter what.