Mikan Yūki is the younger sister of Rito Yūki, an 11-year-old elementary school student who advances to sixth grade. With parents Saibai and Ringo Yūki often absent due to work, Mikan shoulders significant household responsibilities, managing cooking, cleaning, and laundry. This role demands a maturity and independence beyond her years. Despite her youth, she keenly observes interpersonal dynamics, recognizing her brother's feelings for Haruna Sairenji and Kotegawa Yui's affection for him. She frequently teases Rito about his accidental magnetism towards girls, calling him a "playboy."

Her relationship with Rito is layered. Though she rarely uses "Onii-chan" and openly criticizes his clumsiness, she deeply cares for him. Moments like Rito nursing her through illness or giving her Christmas gifts elicit genuine gratitude using the familial term. Vulnerable times, such as pollen-induced intoxication in an OVA, reveal a more childish, clingy side where she fears losing her place in his life. Childhood admiration for Rito stemmed from acts like rescuing a cat or desperately searching for her when they got separated at a crowded pool. While Lala Satalin Deviluke's arrival altered their dynamic, she still values their bond.

Progressing through the series, Mikan forms key relationships outside her family. She builds a close friendship with Yami (Golden Darkness), partly due to their similar apparent ages, finding emotional support and relatable companionship. In contrast, she interacts cautiously with Momo Belia Deviluke, distrusting her "harem plan" for Rito. Mikan actively intervenes to thwart Momo's nighttime advances, citing moral concerns. She also struggles to connect with Mea Kurosaki, Yami's sister, due to differing personalities.

Attending Sainan First Elementary School, Mikan's maturity sets her apart from classmates. Popular among boys, she consistently rejects their advances, sometimes citing Rito as a reason for her disinterest. Her daily life balances school duties with an expanding household management role as more characters join the Yūki home, including caring for the plant-like alien Celine. She possesses astraphobia, a fear of thunderstorms.

Her name "Mikan" (美柑) means "beautiful orange," reflecting the Yūki family's fruit-themed naming tradition—Rito means "pear," their mother Ringo means "apple"—which occasionally becomes a subject of humor. She is often depicted eating blue popsicles and exhibits temporary resistance to Celine's pollen effects, though this resistance weakens around Rito.