As children, Rito and Mikan were very close, and Mikan still called him "Onii-chan". Because their parents were not often at home, Rito looked after Mikan and they spent their time together all the time, walking and playing outside together. Unlike Rito, who has grown up to become very clumsy, awkward and anxious, especially around women, Mikan has become hard-working, confident, and good with housework, far surpassing her older brother's maturity and reliability. She stopped calling him "Onii-chan" and doesn't quite look up to him and respect him the way she had before. In the first episode, she tells Rito that she's embarrassed to be his little sister when she believes that he's having sexual fantasies of girls in the bathtub.
Nevertheless, the two of them are still very close and love each other as brother and sister, even to the point of admiration towards one another. Rito always considers Mikan as his one and only precious little sister, and Mikan is happy taking care of Rito at home. Until Lala's arrival, it was still only the two siblings together at home, and at one point, she felt a little lonely from not spending time with Rito lately. Her friends believe that the reason she rejects many interested boys at school is because she already has a "wonderful Onii-chan". Their impression comes from the fact that Mikan talks about Rito a lot at school, despite mostly calling him a useless big brother who's nothing really great.Mikan may not show it but she cares very deeply for Rito and misses his company to the point one could suspect her of having a brother complex. She seems almost enthralled by his sensitivity and his kindness, and often worries about being seperated from him. This more evident in the second OVA where she wondered if he would marry her if they were not blood related, and in the manga where she blushes every time another character mentions how "great" of a person her brother truly is, even though she claims she doesn't see why he's such a big deal. Her brother complex may be the reason she rarely calls Rito "Onii-chan", less a lack of caring for him and more a sign of her wish that their relationship was something more respectable due to the sisterly reverence she has for him. She also states how she and Rito used to play together before Lala entered their lives. Much like Yui, Mikan's also shown to be extremely fond of Rito's character due to the fact that he's the only genuinely nice boy she knows (as all the boys at her school always try and date her despite her constantly telling them that she doesn't want to).
Even though she says that he's only good at useless things (which was shown a couple of times in the series), Rito has proven to be useful at more life-threatening situations, and in To LOVE-Ru Darkness chapter 31 she admitted that he can be counted on when someone is in trouble.
She sometimes shows a playful side towards Rito, and seems to enjoy teasing him to the point of Momo noting she can become bratty while doing so. Whenever Rito has been turned into his female alter-ego, Riko, she does not hesitate to tease him by offering girl's clothes or refering to him as one would an older sister. It was even seen that when Riko thanked her for helping her, it seemed to flip a switch in her wherein she simply couldn't avoid teasing her "Onee-chan".
Thanks to Momo's plan, Mikan was tricked into taking a bath with Rito again after they haven't done it in years. Since then Mikan and Rito are seen taking a bath together comfortably and organically (in Darkness Chapter 46.5). Mikan has also stopped making it a big deal for Momo to take a bath with Rito (since Darkness Chapter 73).
Similar to how she treats her older brother, Mikan generally acts like an older sister to the childlike Lala. Mikan is somehow extremely casual and unfazed with the eccentricities around Lala, whether it's Rito's recent engagement to Lala, or even Lala's alien origins. Even finding Lala completely naked in Rito's bed has become a normal occurrence to her. Mikan is pleased to know that things will get more interesting upon Lala's arrival in their lives. They also have a close friendship, as shown when they are showering together Mikan offers on scrubbing her back.
Despite this Mikan some times feels jealous of the attention that Rito gives Lala much like she does with the other girls. However as they grow closer this becomes less common.
Lala always enjoys Mikan's cooking.
Mikan is Yami's most significant relationship. Perhaps due to appearing to be the same age, Mikan constantly approaches Yami to encourage a friendship with her. Although Yami was hesitant at first to be acquainted and friendly with anybody, she has also grown attached to Mikan. They are often seen hanging together. Mikan is essentially the only person Yami truly considers as a friend, and has become willing to put her own life at risk to protect her. Despite Yami's emotionless expressions, Mikan says that she can understand how she feels.
Although she at first welcomed Lala's little sisters into her home, Mikan and Momo's relationship has grown humorously adversarial when Rito is involved. Strangely, while she doesn't mind the innocent and childish Lala clinging onto Rito and sleeping naked next to him, Mikan particularly dislikes Momo's more mature doting and more sexual advances with Rito. Although it's not explicitly stated why, it could be because she is somewhat suspicious of the mischievous Momo's intentions. Or more likely it is due to feeling that, unlike Lala, Momo is gradually filling the place in Rito's life which belonged to Mikan. This is how Mikan felt when she reflected on how Momo helps her with the cooking and laundry at home, gradually taking the wife-like position in Rito's life, which had always been occupied by Mikan. But Momo assures her that this was not the case, and that Rito himself expressed how precious Mikan is to him as his little sister. Momo admitted that she envies how Mikan is a close part of Rito's life and tells her that she hopes to become as close to him as Mikan is. Unknown to Mikan, Momo is adding her to the "Harem Plan," intending to include her into the harem Momo is building around Rito, despite Mikan being Rito's sister.
Nevertheless, Mikan has become something of an obstacle to Momo, barring Momo's access to Rito and warning Rito to be careful around Momo. She now acts as a sort of sentry to stop Momo from sneaking into Rito's bed at night. She even becomes uneasy when she is forced to leave Rito and Momo alone together at home, as, after being untimely distracted, she rushes straight home to prevent Momo from doing anything to Rito.
Her dad and a manga artist, who constantly asks her to come along with Rito to bring him manga supplies to his apartment with the bonus of having her clean up and cook food for him and his co-workers which he greatly enjoys, although Mikan finds it tiring. She also hates how he thinks that Momo is as sweet as she acts around him.
Mikan's mother, who works as a fashion designer travelling around the world and as such is rarely home. Unlike Rito, she is not shown to to be embarrassed by her mother's eccentric habits and seems more relaxed in her presence than her brother.
Mikan and Celine get along extremely well and they are seen playing together on some occasions, having developed a near perfect mother-daughter/sisters relationship. Celine also sleeps in Mikan's room. In the anime, Celine would sometimes copy Mikan's motherly personality. It was Mikan who named her Celine.
Classmates and TeacherEdit
Mikan is often praised by those who know her at school, such as her friends and her teacher. She is seen to have at least two female friends at school named Sachi and Mami, but no male friends because they all would like to date her, which she is not interested in.
Her friends praise her for being cute, talented and popular. They are also interested in her love life, as she is quite popular with the boys, but continually rejects their invitations on dates because she is apparently not interested in dating. Because this effectively makes her older brother, Rito, the only boy in her life, and because she apparently talks about him at school, her friends believe that Rito must be a very cool big brother to her and are eager to meet him, which Mikan always dodges from allowing to happen because of the erotic mishaps that constantly happen around him.
Mikan's teacher, Haruko Nitta, is a big fan of Mikan's mangaka father's work, and may have planned a home visit to Mikan's house with the intention to meet him. Rito has said that because Mikan is so mature and reliable, everyone at school, perhaps including her teacher, depend on her.