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.