Kokomi had a good relationship with her brother, but lately it's become bad due to what her brother did to Kusuo and hates her brother as well to never talk to him again. However, like every boys, Kokomi's brother will do anything to protect his little sister. Kokomi is most likely unaware of his incestuous feelings for her, but she does find some of his actions a little bit strange.
Kokomi, being the prettiest girl in school and the whole world, essentially has all of the boys at PK Academy in her pocket since they are willing to do whatever she wants.
However, she has rejected them all, since she's infatuated with him but not the rest of the school.
Kokomi started noticing Kusuo when she met him once downtown. She thought that if she talked to him, his summer would be better because she had noticed him. However, Kusuo completely ignored her. When Kusuo disappered, she thought that she had imagined him, and had "fallen in love with him." Overtime, she realized that she really had fallen in love with Kusuo. Kokomi is shown to so be persistent in getting Kusuo to like her and keeping up her image as the perfect girl that she'd go as far as to eat some gross ramen with Saiki. Kusuo admires her dedication the keeping up her "perfect girl" persona. She has extremely good luck, like when she runs into Kusuo's friends, which leads her to spend sometime at his house. She claims this is because "god loves her." During a New Year's dinner at Kusuo's house, her memories were altered so she thought that Kusuo had bad past experiences with girls. Kokomi once brought chocolates for Kusuo but wasn't able to give them to him because she was afraid he’d get beaten up by jealous guys, so she throws it away. Kusuo grabs it anyway, because he forsaw that she was going to throw perfectly good candy out the window. She has also caused his powers to go haywire twice, once when she removed Kusuo's limiter when they were on a school trip, and once when they he was seasick, which got them stranded on an island.
Kokomi and Chiyo used to have a love rivalry about Kusuo however, after some misunderstanding caused by Kusuo's powers, he was able to mend their friendship and the two got closer. They often times try to help the other with their crush (Kokomi's crush on Kusuo and Chiyo's crush on Shun). They call each other by their given names, indicating a close friendship.
Kokomi and Imu have a strong friendship, but Imu secretly has a crush on Kokomi. They call each other by their given names, indicating closeness.
Kokomi met Kumagoro by chance when he lost his wife Kumi somewhere in the city. After making a call about his lost wife, Kumagoro was sitting dejectedly in front of the telephone post, so Kokomi came and talked to him. He thought she was an angel that had come to save him. After telling her the problem, she then lends him a hand and asked for the name of the house Kumagoro was looking for. After learning it was the 'Saiki' house, Kokomi realized that he was actually Kusuo's grandfather. The meeting of the two worried Kusuo but he later stopped following the two of them because it was unbearable. At the house, Kumi casted her vote for Mikoto as her granddaughter-in-law, but Kumagoro wasn't keen to her idea as he wanted Kokomi to marry Kusuo.