Yūki Mikan

Mikan Yuuki is Rito's independent and reliable 12 year-old sister. Mikan takes care of the housework while their parents are away on business and watches after Celine. In contrast to her brother, she is very mature and does not hesitate to tease him, going as far as stating they aren't blood-related, but quickly retracting the statement, playing it off as a lie. Mikan also seems to have a certain lack of respect for Rito because of this (she hardly ever addresses him by his honorific as an older brother), but does answer him "Onii-chan" as thanks for trying so hard to take care of her when she develops a fever and when he gives her a Christmas present.

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, 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 her brother, also in the 5th ova where after being hit by the pollen she tries to molest him while affectionately calling, "Onii-chan". Her brother complex may be the reason she rarely calls Rito "Onii-chan", less a lack of respect and more a sign of her wish that their relationship was something other than that of a brother and sister. She also states how she and Rito used to play together before Lala entered their lives.

It is later revealed that Mikan is terrified of thunder and lightning, though Rito would always comfort her. Mikan has grown quite attached to Golden Darkness, perhaps due to Darkness seemingly being the same age as her. Mikan is very perceptive and seems to be able to read people's emotions, even their reactions, most of which is Rito's obvious anxiety around Haruna Sairenji and Yui Kotegawa's subtle feelings for him. Mikan even calls him a "playboy" (so does Nana) for attracting so many girls all of a sudden. Mikan, however, does respect Rito's relationship with Lala, even going so far as to tell Momo not to get too close to Rito for Lala's sake. Although it is fairly obvious that she could have done this out of jealousy from how close Momo was getting to Rito.

Relationships with Other Characters
Rito Yuuki 

As children, Rito and Mikan were very close, and Mikan often 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.

Nevertheless, the two of them are still very close. 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.

Lala Satalin Deviluke 

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 occurence to her. Mikan is pleased to know that things will get more interesting upon Lala's arrival in their lives.

Lala always enjoys Mikan's cooking.

Golden Darkness 

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 aquainted 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.

Momo Velia Deviluke 

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 has a particular dislike of 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. 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.

Trivia
Mikan's cameo appearance with Celine in chapter 1 page 37 in the manga Mayoi Neko Overrun!



And again in chapter 3 with her friends in page 26.