Runa Yomozuki is a member of the Hyakkaou Private Academy Student Council and leads the Election Committee, acting as a referee during official gambling matches. She oversees election events, ensures neutrality, and enforces game rules throughout various story arcs. Standing 130 cm (4'3") with a petite, youthful appearance, she has light skin, long platinum blonde hair featuring short bangs parted at the forehead, and purple eyes. Her typical attire consists of an orange rabbit-themed kigurumi hoodie worn over the standard academy uniform: a red blazer with black trim, a white button-up shirt, a dark pleated skirt, a tie, black-and-white striped stockings, and brown loafers. She frequently sports light pink-painted fingernails and orange lip gloss, and is almost always seen with lollipops or playing handheld gaming devices like a pink PlayStation Vita or Nintendo Switch.

Publicly, Runa exhibits a bubbly, charismatic, and carefree demeanor, appearing enthusiastic during broadcasts yet showing disinterest in non-gambling academy affairs. She often becomes distracted by video games or snacks during Student Council meetings. Beneath this cheerful facade, she demonstrates cruelty and disregard for human suffering, mocking Mary Saotome about forced marriage plans and showing amusement during Midari Ikishima's self-mutilation. She supports the academy's oppressive "housepet" system and uses intimidation to enforce compliance, but fiercely upholds fairness during gambles, punishing cheating and maintaining impartiality as Election Committee head. Her neutrality faces occasional tests, particularly when Student Council interests conflict with her duties.

In early storylines, she assists President Kirari Momobami by relaying information, such as Yumeko Jabami's challenge to Kirari and details of Mary Saotome's life plan. She likely sabotaged Kaede Manyuda by sending Yumemi Yumemite's torn fan letter to Yumeko, aiming to undermine both characters simultaneously, an action implied to align with Kirari's interests. During the Tarot game involving Yumeko and Kirari, she served as dealer, confiscating a marked card to prevent outcome scrutiny and later reporting events to Ririka Momobami. Her surprise at Kirari's true identity during a concert highlights her occasional exclusion from the Momobami twins' schemes.

During the Election arc, she oversees the entire process, announcing frontrunners and explaining rules under the committee's motto of "absolute neutrality." She referees critical matches like the Nim-Type Zero game involving Yumeko, Ryota Suzui, Miyo Inbami, and Miri Yobami, where she allowed Ryota to "accidentally" drop cards to circumvent rules against sharing information. She also hosted the singing competition between Yumemi Yumemite and Kawaru Natari, devising the tiebreaker game for the final round. Her neutrality encountered challenges when the Momobami clan targeted Student Council members, leading her to intervene and validate a gamble after Rin Obami's deception to restore the committee's credibility. After Kirari's abduction, she joined the new Student Council tasked with retrieval but stated indifference toward Kirari's fate, emphasizing her transactional loyalty.

In the spin-off *Kakegurui Twin*, set before the main series, she is depicted as friends with Aoi Mibuomi, engaging in video games and light teasing. She expresses excitement about school festivals and dispatches observers like Inaho Yamato to monitor notable gambles. Another spin-off, *Kakegurui (Kakkokari)*, reveals her self-consciousness about her childish appearance, particularly when treated condescendingly by younger Student Council members.

Her surname, Yomozuki, combines the kanji for "underworld" (黄泉) and "moon" (月). Her first name, written in hiragana (るな), carries no inherent meaning but phonetically evokes "luna" (moon). She is one of few characters with a hiragana-only given name, and her name is occasionally misspelled as "Yomotsuki".