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. After Yuzu revealed she loves Mei not as family but as a lover, she respected it and gave Yuzu her full support of her best friend feeling's towards Mei.
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. Later on she revealed that she was in love with Mei. 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 passes Harumi starts to trust Matsuri to a degree that they're on friendly terms and are usually 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 Harumi is somewhat annoyed by her fantasy of Haruyuzu.
Shiraho is Harumi's senpai.
Not much is known about their relationship, but Harumi slightly fearful of Mitsuko. Harumi becomes worried when her sister asks about Yuzu. She doesn't like how Mitsuko forces people make decisions, that work in her favours her. After the election their relationship has started to improve for the better.
Apparently, Harumi and her sister live with her grandmother.