In part 1, Miuna was envious of Sayu and was glad when they became friends. Miuna didn't care when bullies began to pick on her, which confused Sayu and lead Sayu to talking with her and the two becoming best friend. In the beginning, Miuna relied on Sayu for a lot of things, one of which was talking for her.
In part 2, Miuna isn't a very good friend to Sayu to which she admits and apologies for. Miuna invites Sayu to go to town with her and Hikari in hopes it wouldn't freak Hikari out so much. Miuna ignores Sayu during this time and when Sayu witness Miuna getting bratty and tries to make Miuna see how wrong she was, the two got into a serious fight that resulted in Miuna being un-reasonable and running away. Later, when Miuna sees Sayu again, both apologies together and make up. Later, Miuna hears Sayu speaking about her feeling for Kaname and how she thinks it's wrong to still like him, this makes Miuna upset and she confronts Sayu about causing the two to fight once more. Miuna takes Sayu for granted. Sayu is the only other person other then Hikari to see how childish and selfish Miuna is, she is also the only person who would confront her about it.
In part 1, Hikari states that Miuna is a annoying little girl that likes to play dirty with her friend, Sayu. Through out part 1, Hikari helps Miuna and soon learns that she is the daughter of the man his sister is in love with and starts to form a 'brother' relationship with her. He even takes her downtown to help find a gift for his sister with her to which the others came along as well. It shown that Hikari cares greatly for her as a younger sister as he teases her like a brother would do. Nearing the end of part 1 Miuna helps Hikari and friends with their projects.
In part 2, it is revealed that Miuna has developed unrequired love for Hikari. She is now 14 years old, the same age and is happy about it until she realizes that Hikari is falling into depression and began to worry for him. Hikari holds no romantic feelings towards her, which she does not care for since she understand he wouldn't ever leave her and will always be the older 'brother' she needed. As she admits to Sayu, she won't change her feelings forcefully, they will change willing if needed, but for now she will support him and carry on loving him as she helped him to save Manaka. She also wants to protect his smile, despite knowing it was fake with Manaka not being around.
Miuna is envious of Manaka for Hikari holds feelings for her. At one point, Miuna isn't sure whether she doesn't want Manaka to wake up . But then helps Manaka come back and helped her after she came. And even become the pride instead of her. They soon become close friends.