Nagatsuki Yoru serves as the male September representative for TSUKINO TALENT PRODUCTION and a member of idol unit Procellarum. With black hair, blue eyes, and narrow sloped shoulders favoring ladies’ clothing, his appearance complements a personality defined by earnest sincerity, meticulous preparation, and a mature aversion to impulsivity. Dubbed "Procellarum’s Mother," he embodies nurturing instincts honed through childhood cooking sessions with his mother, whose influence also shaped his feminine grace and culinary expertise. Beneath his composed demeanor lies occasional stubbornness and an offbeat humor triggered by poorly delivered jokes.

An only child, Yoru forged closeness with his foreign-company-employed mother through shared meals, while details of his father’s work remain private. His entry into idolhood began when he and childhood friend Haduki You were scouted during a Tokyo excursion. Their bond thrives on mutual dependence: You relishes Yoru’s dishes, while Yoru habitually mends their disputes, even when You errs. Though their contrasting temperaments spark friction—evident during Full Moon Festival rehearsals—their enduring partnership withstands tensions, anchored by candid communication.

Yoru’s modesty fuels a self-view as unremarkable among peers, initially causing photo-shy hesitance before gradual acclimation. He counters his pessimism by seeking guidance from upbeat colleague Shimotsuki Shun. At Procellarum’s dorm, culinary synergy with Satsuki Aoi earns them the "mother-combi" title, collaborating on meals during leisure.

Notable traits include 0.6 visual acuity managed without corrective lenses due to discomfort and a propensity for seasickness. His name, Nagatsuki Yoru, intertwines the archaic September term "nagatsuki" (linked to lengthening nights) with "Yoru" (night), reflecting his moonlit birth. This duality resonates linguistically through "yonagatsuki," reinforcing September’s thematic tie to nocturnal expanse.