Yuzu is Mei's new stepsister, who is a month older than her. At first, Mei sees Yuzu only as a nuisance and makes in the hopes of shutting her up and repelling her away, yet begins to become attracted to Yuzu's brash personality. In later chapters, Mei appears to be reciprocating Yuzu's romantic feelings to some degree, becoming rather possessive of her due to Matsuri's advances on Yuzu opening up to her, and feeling guilty when she believes Yuzu is troubled or sad because of her actions. She expresses to Matsuri she's thankful for having met Yuzu.
However she completely rejects Yuzu when she wants to confess, as Yuzu didn't infere her feelings clearly enough for Mei to understand. After both confessing their true feelings for each other she starts dating Yuzu. Mei constantly worries about Yuzu and was concerned why Yuzu came home crying, after hearing it was because she had found out not everyone will approve of their relationship, Mei writes in Yuzu's dating notebook to encourage her by saying no matter what hardship may happen in the future, it shouldn't stop you from being who you are. She subconsciously cried tears of joy when Yuzu put the ring she had got for her and since then Mei tries to wear the ring whenever she has the chance.
In Chapter 36, Mei left a letter for Yuzu, it stated how she had felt like she had hurt Yuzu, and left the Aihara household to marry her fiancé and run the Aihara family, leaving Yuzu in a emotional wreck. In Chapter 41, she said she had been hurt deeply by betraying her and trying to rush the engagement so she could stay away from her, even going as far as saying that she hates her. Yuzu then engaged to her, to which Mei accepted.
As of the end of the original manga run, the two characters are now married.
Mei's father, who also played the part of her sensei and role model. She aspired to be just like him. After he left home to travel, the burden of taking care of the school in his absence was lain upon her shoulders, and she hoped he'd return to become the principal after her grandfather. Although she wishes to better her relationship with her father, she struggles to accept who he has become. However, she realized he didn't want that anymore and feels she has nothing left living for as taking care of the school for the sake of her father had been her assumed purpose in life, started to distance herself from him.
However, Yuzu's influence leads her to begin repairing the damaged relationship with her father, as well as to search for her own purpose in life. She has a habit of calling him "sensei", as he was more of a teacher to her than a father, which annoys him, but at the airport thanks to Yuzu's encouragement she called him father for the first time.
Mei looks up to her grandfather. After he was hospitalized, she decides to take on his workload on her own, which, along with her work at the Student Council president, causes severe stress and fatigue to her. When Mei and Yuzu kiss in the chairman's office, Mei puts his portrait face down as a sign of respect to him.
Yuzu's mother immediately accepted Mei into their family and treats Mei no different to how she treats Yuzu. Yuzu's mother genuinely cares and worries for Mei, and as a result, Mei has taken a liking to her.
Himeko is Mei's vice-president in the Student Council and her childhood friend. When they were younger, they used to play with each other, but after Mei's father left, she started to distance herself from Himeko, starting to see her as a nuisance rather than a friend, but Yuzu helped them rekindle their friendship. Mei is the subject of Himeko's affections, but these emotions are not reciprocated.
Harumi is Yuzu's best friend, but she and Mei rarely interact, so their relationship is unknown.
Glasses-Senpai is a fellow member of the school council as well as Mei's second vice president. They are on friendly terms with one another and often work together to reprehend delinquents like Yuzu.
Mei's first encounter with Matsuri was witnessing her kiss Yuzu. This leads her to develop and instant dislike of Matsuri, who threatens Mei to leave Yuzu alone. Mei notes that the two of them aren't so different. Matsuri realizes that Mei never wanted to take Yuzu from her and realized even Mei cares for her feelings too, all conflict gets resolved and eventually Matsuri stopped thinking about her as a rival and starts seeing Mei as another older sister. they both get along as Yuzu's younger sisters. Mei tutors Matsuri with her studies from time to time.
They dated for only a short period of time, but they quickly broke up after Sara found out the connection between Mei and Yuzu. Mei later apologized to Sara, saying she was just using her and didn't want to lose her as a friend because of that, Sara said she'd forgive her if she and Yuzu held hands on the way back home, which they did.
When Nene just started she had dyed her hair chestnut like Yuzu and was confronted by Mei and the other members of the student council and was almost expelled when Mitsuko came to check on the school and forced a special student council election. She later confronted Nene to find out why she looked up to Yuzu and appologized when after words because she felt like she was threatened by her. they seem to be on good terms after.
Suzuran is Mei's senpai and have never met prior to the festival but Shiraho had followed Mei around at school because she could not read her like the other students. After seeing her at the festival with everyone else she immediatly noticed that Mei was a different person and confessed her feeling to her when she was "dumped" by Mei she became willing to keep Yuzu's and Mei's love a secret and help it blossom. It was thanks to her Mei now has a better understanding of her feelings towards Yuzu. Since than they maintain being friends having a senior junior relationship.
They never directly interacted, but Nina tried to keep Yuzu from intruding in her relationship with Sara, if only for the sake of Sara's happiness.
Mei appears to respect Mitsuko a lot, even though Mitsuko scolds Mei due to Yuzu's breaking of the rules.