Yuzu is Mei's new stepsister, who is a month older than her. At first, Mei sees Yuzu only as a nuisance and makes out with her in the hopes of shutting her up and repelling her away, showing her no respect as she defies the school's rules and even refuses to acknowledge her as her sister, yet warms up to her once she cancels her arranged marriage to Amamiya. In time, she begins to become attracted to Yuzu's brash personality. At first, Mei tried to suppress this by hypocritically stating they are sisters and girls but Yuzu's subsequent forceful kiss along with her helping mend her relationship with her father causes her to acknowledge her feelings at last and starts reciprocating Yuzu's romantic feelings to some degree, becoming rather possessive of her due to Matsuri's advances on Yuzu and 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, Yuzu rejecting her offer for sex and not conferring her feelings clearly enough for Mei to understand causes her to completely reject Yuzu and hurt, she tried to have a relationship with Sara but 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.
However, unfortunately for Mei, her grandfather had once again put her in an arranged marriage with Udagawa, which leaves her distraught as she will have to break it up with Yuzu. This results in Mei hesitating to have sex with Yuzu, tearfully nearly telling Yuzu she loves her had Matsuri not screamed in outrage, and then crying when trying on dresses. Initially, Mei tried to fulfill her obligation, as shown in Chapter 36, where Mei left a letter for Yuzu, stating she felt that she had hurt Yuzu, and left the Aihara household to marry her fiancé and run the Aihara family, but ensured her that she was deeply grateful of the time they spent and promised to pray for her happiness, leaving Yuzu an emotional wreck. However, she was truly deeply remorseful and hurt by having to betray her, which she initially tried to cover up by taking as much work as possible and distancing herself from Yuzu in school and even quitting school to rush the engagement so she could stay away from her, but when Yuzu finally confronted her, Mei finally broke after saying that she hates her, stating that no matter how hard she tried, she just couldn't stop herself from thinking about her. No longer able to deny her feelings for Yuzu, she quit the engagement when Yuzu proposed to her, to which Mei accepted.
As of the end of the original manga run, the two characters are now married.
As shown in Citrus Plus, Mei has become content and determined to ensure her relationship with Yuzu remains firm. She states that while she still lusts for Yuzu, she wants to sincerely respond by following the steps from the very beginning and even trusts Yuzu enough to let her tell their friends about it, though she is still far from being willing to disclose intimate parts, as she quickly became angry when Yuzu told too much. She is touched by Yuzu's wish to go to the same university as her but also concerned when she focuses too much on studying, asking Harumi to tell her to stop and even going as far as to pardon Yuzu and Harumi for breaking the rules by going out still with school uniform when Yuzu returned to herself. In addition, instead of letting Yuzu deal with Sayaka knowing of their love or dealing it herself, she instead works together with her to solve it, stating she no longer wants to have to do things herself or let Yuzu do things by her own, showing her improved maturity.
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 laid upon her shoulders, and she hoped he'd return to become the principal after her grandfather. She was still fond of him to the point where Yuzu stating how her father should just take over and come back already caused her to smile. However, when her father came back and revealed he didn't want that anymore, Mei was devastated and feels she has nothing left to be living for, as taking care of the school for the sake of her father had been her assumed purpose in life, so she started to distance herself from him. She has a habit of calling him "sensei", as he was more of a teacher to her than a father, which annoys him.
However, Yuzu's influence allows her to begin repairing the damaged relationship with her father, as well as to search for her own purpose in life. At the airport thanks to Yuzu's encouragement she called him father for the first time. Mei stated that she truly respects him when her grandfather regrets not raising Shou to be a better successor, feeling pride in how even though he has left the academy, he still teaches, proving he is definitely an Aihara. When Sho helps her ensure her relationship with Yuzu will go on, Mei is genuinely grateful for his help and glad that she can approve of his ways, though she doesn't want his advices in love, even giving him a glare over it, and appears to be still a bit awkward around him, blushing when her father praises her for her performance in the meeting.
Mei looks up to her grandfather. After he was hospitalized, she decides to take on his workload on herself, which, along with her work as the Student Council president, causes severe stress and fatigue . 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, their relationship became strained, revolving more around cold professionalism, as Mei started 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. Regardless, Mei appreciates Himeko's help and wishes she will always remain a dear person to her.
Harumi is Yuzu's best friend, unlike most girls in her school, Harumi isn't interested in fawning over Mei but she and Mei have a cordial relationship, though she dislikes her attitude. Harumi knows that Mei and Yuzu are stepsisters, but wasn't aware of Yuzu's crush on Mei or that Mei is currently dating Yuzu. Later on she finds out that Mei is in relationship with Yuzu, and supports their relationship. Mei in return trusted Harumi with telling Yuzu to stop over-studying for her sake and even felt grateful enough when Harumi did it for her that she let Harumi off without any punishment for breaking the dress code of the academy.
Kayo is Mei's Senpai and 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.
They never directly interacted, Sara believed Mei was her predestined person and was hoping to bring them together however it fails in the end. But they ended up being friends afterwards.
When Nene just started she had dyed her hair blonde 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 apologized afterwards because she felt like she was threatened by her. They seem to be on good terms after that.
Suzuran is Mei's senpai and have never met prior to the festival but Suzuran 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 immediately noticed that Mei was a different person and confessed her feelings to her when she was "dumped" by Mei, she then became willing to keep Yuzu's and Mei's love a secret and help it blossom. It was thanks to her that Mei now has a better understanding of her feelings towards Yuzu. Since than they maintain being friends having a senior junior relationship.
Mei appears to respect Mitsuko a lot, even though Mitsuko scolds Mei due to Yuzu's breaking of the rules.
Mei is her senpai and both are part of the student council. She seems to have a strained relationship with her.