Yūki Mikan

Mikan Yuuki (結城 美柑) is Rito's independent and reliable 12-year-old sister. Mikan takes care of most of the household work while their parents are away working and also watches over Celine. In contrast to her brother, she is very mature and does not hesitate to tease him, going as far as lying that they aren't blood-related, but quickly retracting the statement and telling the truth. Mikan also seems to have a certain lack of respect for Rito because of this (hardly ever addresses him by 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 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 (even though she attracts so many boys at her school). 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's possible that she could have done this out of jealousy from how close Momo was getting to Rito.

Mikan is the only main character that doesn't attend Sainan High.

Appearance
Mikan has gold brown eyes and brown hair, she is described to be really cute by some characters. According to a "Full size Dakimakura Cover-Mikan Version", her measurements are B70-W52-H73.

Personality
As a younger sister, Mikan is widely known for her maturity and reliability for her age. Her maturity is also quite unique, as while she is Rito's younger sister and the youngest character in the cast, she is arguably more adult than most of the other characters, and basically acts as a big sister, wife, or mother to Rito and all who live in her house. Because she is very good at cooking and housework, she essentially takes care of her big brother. Rather than looking up to him, she instead treats him like he is her little brother, and rarely calls him "Onii-chan", as she used to. Mikan generally treats Lala the same way, enjoying her company like a little sister and pleased to see her brother's life becoming more entertaining upon her arrival.

Like most girls in the cast, she is also aware of her breast size (as shown in To Love-Ru Darkness Chapter 56) and often feels inferior when comparing other girls' to hers, mostly Lala, although her physique is quite normal for her age. Despite this, she still feels insecure about this, and has developed a positive connection with Nana due to their similar chest-related angst.

In the fifth OVA, when Mikan was effected by the pollen of one of Momo plants that makes anyone who inhales it to act freely, she acted childish and clingy towards Rito, even calling him "Onii-chan", showing how shy she is at expressing how she feels towards Rito and that she just acts mature around others. She even feared that her place in Rito's might slip away when Momo was being too close to him.

Mikan is very perceptive, as she is good at understanding other people and noticing their feelings. She knew very quickly her brother's crush on Haruna, and Haruna's similar feelings for Rito as well. And she knows that Rito is too scared to enjoy perverted things, and that Lala is innocent and quite childish, so she doesn't mind catching Lala naked in her brother's bed in the morning and even treats it like something normal, though it should be noted that she overreacted once when she mistook them having sex in his room, when Lala was just moaning because Rito's hand was stuck to her tail. But she particularly doesn't like Momo sneaking into his room at night and sleeping with him, because of how sexual she can be towards him.

Like a big sister or mother, she would try to protect Rito from sexual temptations, and doesn't like it when girls with more perverted tendencies like Momo and Mea would do perverted things to him. She tries as often as she can to watch out whenever Momo gets the opportunity to be alone with Rito, and even guards his room at night to stop her from sneaking in and sleeping next to him. She also doesn't particularly like Mea when first meeting her, and though she tried to get to know Mea better after learning that she is Yami's sister, she finds that she cannot deal with her type and likely can't become close friends with her as she is with Yami.

Mikan seems to get along quite well with characters who are more level-headed, composed, and mature, such as Golden Darkness (Yami) and Rin Kujou. In To LOVE-Ru Darkness, despite being older than even Mikan's big brother, Rin seems to have become friends with her, and is astonished that someone like Mikan is the younger sister of the clumsy and quirky Rito. And Mikan is the only person whom Yami truly considers to be her friend. Rito explains that because she is so mature and reliable for her age, deep down, Mikan can get lonely without people she can relate with, and so she can use older friends who are like big sister figures to her, whom she can lean on (To LOVE-Ru Darkness chapter 5).

Mikan has a case of Astraphobia, the fear of thunderstorms.

History
Prior to the series, Mikan had always looked up to her brother and often called him "Onii-Chan" because of how kind and dependable he always seemed, such as climbing a tall tree to save a cat. Due to their parents hardly being at home, it was always just the two of them together, even during Christmas, and playing on snowy days. Over time her respect for Rito seems to lighten however she still likes how he is always caring for her. For example, when she was 9 years old, Rito took her to a swimming pool where it was very crowded at the time and she got seperated from Rito before she knew it, when he found her she noticed how desperately he searched for her from seeing his shirt full of sweat. Although she thought it was stupid of him, she was also happy knowing how much he really cares for her.

Trivia

 * She is often seen having a blue ice lolly.
 * Like many other anime girls (and some guys), Mikan entered Saimoe Contests.
 * ​To see her result, visit here: http://saimoe.wikia.com/wiki/Yuuki_Mikan
 * To vote for her when the match is on, visit here:  http://www.internationalsaimoe.com/voting/
 * Her name Mikan is derived from the fruit Orange (Her name in Kanji (美柑) is translated literally as "Beautiful Orange"), which the manga and anime both acknowledge and make fun of.
 * This also made Saki and her friends misunderstand this name when Rito and Lala went to her family's supermarket. Initially believed "Mikan" to be some kind of orange, they issued Lala and Rito to a challenge and they two must beat Saki in order to continue searching.
 * Yuuki siblings were named after fruits, which are Pear (梨) and Orange (柑) for Rito and Mikan, respectively. Comically, their mother's name was after a fruit (apple) too.
 * Mikan makes a cameo appearance with Celine in Chapter 1 page 37 in the manga Mayoi Neko Overrun!, more than any other caracter in the series.