Shinobu Sengoku is a student at Yumenosaki Academy and the RYUSEI Yellow member of the idol unit RYUSEITAI under STARMAKER PRODUCTION. Significant social isolation during primary and secondary school led to intense social anxiety, causing him to identify with solitary ninjas and cultivate a "chuunibyou" persona centered around ninja admiration. This loneliness and anxiety made peers perceive him as strange, worsening his tendency to distance himself. Physically, Shinobu is short with an average build. He has dark purple hair featuring two yellow streaks in his bangs and a small yellow section at the back of his head. His bangs typically cover his right eye, and his eyes are light gold. His casual attire consists of the Yumenosaki uniform with an unbuttoned collar, a blue tie, a black undershirt, and a purple jacket beneath the blazer. For performances, he wears RYUSEITAI's yellow-themed stage outfit: a black-and-white leather jacket with yellow accents, gold detailing, and a galaxy-patterned purple interior, paired with white pants featuring a purple stripe, a star-adorned belt, tall yellow boots, and black fingerless gloves. Personality-wise, Shinobu is initially shy and timid around strangers, which he terms his "Shyness Skill." He struggles with crowds and unexpected social interactions, often fleeing or feeling nauseated in group settings. He gradually opens up to those showing kindness, revealing a bright, caring, and pragmatic nature. He becomes deeply attached to supportive individuals, reciprocating their kindness. Friends describe his presence as "healing," though he remains anxious about being disliked or burdensome due to past isolation. He refuses to compromise his ninja-centric identity for acceptance, seeking validation for his true self. At Yumenosaki, Shinobu founded the Ninja Association to fulfill club requirements, as no existing club aligned with his interests. Though initially a one-member association lacking official status, he trained diligently on the school rooftop to recruit members and achieve recognition. His training involved acrobatics, stealth drills, leg-strengthening exercises, and carrying ninja tools like rubber shuriken and provisions, sometimes causing small fires that drew teacher supervision. Through RYUSEITAI and friendships with unit members like Chiaki Morisawa and classmates like Yuta Aoi—who shared his social struggles—he overcame much of his anxiety. This growth allowed his innate positivity to flourish, making him a supportive figure within his unit. As RYUSEI Yellow, Shinobu performs with energetic and acrobatic choreography, leveraging his agility. His stage catchphrase is "Yellow flames are the proof of hope! A lone miracle illuminating the darkness! Ryusei Yellow! Shinobu Sengoku!" His idol activities focus on spreading ninja culture and bringing joy to audiences, though he occasionally debates balancing his solo ninja identity with group efforts. His dedication to RYUSEITAI strengthens over time, and he views his unitmates as essential allies in achieving his dreams. Shinobu's development progresses from a socially anxious loner to a more confident idol. By his second year, he actively supports peers, assists with tasks like finding lost items, and participates in unit collaborations. His dream evolves into using idol performances to showcase ninja traditions, emphasizing sincerity and effort in his training. He retains trademark traits like ending sentences with "de gozaru," a speech pattern inspired by the Sengoku era, maintaining pet frogs named Momocchi and Thunder, and catching insects to feed them, reflecting his unwavering devotion to ninja aesthetics.