Rito Yuuki's little sister.
Mikan lives alone with Rito in the house their parents bought while both of them are away pursuing their respective jobs. The mother as a fashion designer, the father as a manga artist.
Mikan is the sibling that does the majority of the housework, though Rito could do it if pressed, Mikan actually likes to do it. Especially the cooking, Lala always comments on even something as simple as rice. The simple reason for this is that Lala cannot cook anything even remotely edible. Mikan does consent to teaching Lala to cook eventually... but it doesn't help, whatever Lala ends up making afterward is always too spicy to be eaten by humans.
Mikan is terrified of thunderstorms, a fact illustrated by her terrified rushes into Rito's arms. Later in the series, when Mikan thinks that one of Lala's younger sisters is developing an unhealthy attraction to Rito, Mikan sneaks into Rito's room and waits, although she falls asleep and is awoken in the morning by Rito's horrified screams. Making Mikan one of the three women (or girls) to sleep in the same bed with him.
Whenever there is something odd or ecchi occurring in Rito's room, Mikan, without fail, always opens the door and sees said "interactions". Usually Rito waking up to find a naked Lala in his bed.
Possibly due to the appearance of similar age, Mikan develops a strong friendship with Golden Darkness, or Yami-chan as most of the cast calls her eventually. She tries to help Golden Darkness become more "at home" on Earth with shopping trips, fine dining, and other basic activities.