Harumi and Yuzu quickly became good friends, due to their similar interests and mentalities. Harumi is nice to Yuzu and is frequently trying to cheer her up when she's down. Harumi tries to protect Yuzu and keep her away from problems, being especially alert of Matsuri, and her sister Mitsuko.
Unlike most girls in her school, Harumi isn't interested in fawning over Mei. However, they seem to have a rather friendly relationship and don't dislike each other. Harumi knows that Mei and Yuzu are stepsisters, but doesn't seem to realize Yuzu's crush on Mei, about Mei's true self, or about the fact that Mei is currently dating Yuzu.
When Harumi first meets Matsuri, she is untrustful of her, and becomes wary of letting Yuzu alone with her. Matsuri, on the other hand, seems to like Harumi, and enjoys teasing her. Matsuri notices Harumi usually watches things from a distance, and says that it's because she's afraid of getting hurt, and even her "gyaru" persona is part of that feeling. As time pass Harumi starts to trust Matsuri to a degree they on friendly terms sicne they are useally seen hanging out together though they do argue from time to time in a comedic fashion.
Himeko is annoyed by Harumi and her attitude, but Harumi seems to like Himeko, and frequently teases her. During the field trip to Kyoto, Harumi and Himeko hang out together, such as when the class goes to a match-making shrine, and Harumi even drags Himeko to a tour around town, against Himeko's efforts.
They seem to be on friendly terms with each other, but is somewhat annoyed by her fantasy of Haruyuzu.
Shiraho is Harumi's senpai they seem to be on good terms with each other.
Apparently, Harumi and her sister live with her grandmother.