Mitsuki Mashiba is the 26-year-old younger twin sister of Youhei Mashiba, with whom she shares a contentious relationship, frequently clashing during their interactions. She works professionally as an insurance saleswoman and often emphasizes her respected societal status. Her appearance features brown hair styled in a side ponytail, brown eyes, and red business attire. Mitsuki has a significant past as a former member of Kyouko Shirafuji's juvenile delinquent gang, having joined after witnessing Kyouko's strength in defeating her brother. She remains deeply loyal to Kyouko Shirafuji and readily assists at the Wagnaria restaurant when summoned, despite actively distancing herself from her delinquent history. She carries a fist knuckle for self-defense, a remnant from her gang involvement. She is easily embarrassed by her former delinquent appearance—puffy, blonde-dyed hair and yankee-style schoolgirl clothing—and becomes irritated when her brother circulates old photos of her. Mitsuki is highly protective of Yachiyo Todoroki, leading her to dislike Jun Satou due to his romantic interest in Yachiyo. Jun Satou notes her overprotectiveness as a factor hindering Yachiyo's development toward self-sufficiency. Her given name Mitsuki uses the kanji for "beautiful" (美) and "moon" (月). Her surname Mashiba combines "real, genuine" (真) and "brush, firewood" (柴).