Sei Satomi reigns as the top otokoyaku star of the Kouka Revue's Winter Troupe, an all-female theatrical company. Born Yasuko Yabe, she initially dreamed of musumeyaku roles. A significant height increase during her training at the Kouka School of Musical and Theatrical Arts steered her towards otokoyaku specialization, where she ascended to top-star status.

Her stage name blends "Sei," from her real name Yasuko, meaning "star," and "Satomi," referencing her mother's hometown of Satomi Village, translating to "beautiful village." On stage, she crafts a dark, intense persona renowned for "yandere" acting—marked by obsession and volatility—earning her the moniker "The Ultimate Yandere." This performance starkly contrasts her off-stage self: naturally feminine, gentle, fond of cute aesthetics like pannier dresses, and notably clumsy, making her the most accident-prone among Kouka's top stars.

Her career centers on her professional partnership with Ruri Shirohana, the Winter Troupe's top musumeyaku. Though their exact age difference is unspecified, Ruri is acknowledged as older. Sei debuted as top star playing the Phantom in "The Phantom of the Opera," a role chosen due to her deep admiration for Mamoru Ando, whom she considers the character's quintessential performer.

During her Kouka student days, she maintained a traditionally feminine look, often styling her hair in twin tails. She mentors younger students like Sarasa Watanabe, once helping Sarasa overcome creative uncertainty at a sports festival. Sei demonstrated how to authentically "perform as oneself," using a kabedon (wall pin) as a theatrical gesture to emphasize her point.

Born August 31st, she is a Virgo. Standing approximately 1.78 meters tall, comparable to Sarasa, she once speculated her stage name would have been Seira Satome had she pursued musumeyaku roles.