Benio Shinada, also known by her secret alias Scarlet Kiss, is a student at Southern Cross High School and a figure defined by the dual responsibilities of her public and private lives. As a second-year student, she holds the prominent positions of the school's dormitory supervisor and the captain of the kendo club. In this public role, she is known for her skill with a sword, having won prizes in local competitions, and she maintains order within the student dormitories.

Beneath this official persona, Benio is a central operative of the secret organization known as the Glittering Crux, where she serves as the leader of the fifth unit, Filament. Her organization code name is Scarlet Kiss, a direct reference to her unique First Phase ability, which allows her to control any man she kisses. She wields the Cybody named Peshent, which is equipped with the Star Sword Rubis. Benio's involvement with the Glittering Crux is deeply personal; Filament is composed of individuals from families who once bore the marks necessary to pilot Cybodies but have since lost that power over generations. Her motivation is to restore her family's lost honor and to gain true power, a desire that has shaped her personality and actions.

Driven by a profound admiration for strength, Benio has harbored a long-standing affection for her classmate, Newdrige Shindo Sugata, since he defeated her in a kendo match during their childhood. She believes that only strong individuals have the power to change the world, and she sees that quality in Sugata. This admiration, however, is tinged with frustration, as his status as a bearer of the King's Mark makes him seem unattainable. Her role in the story is largely defined by her antagonistic pursuit of the protagonist, Takuto Tsunashi. She initially seeks to defeat him to prove her own strength and regain her family's standing. In a significant early battle, she attempts to use her ability to control Sugata and absorb his powerful Cybody, Samekh, into her own, but the plan backfires when Sugata breaks free of her control and dominates her Cybody instead, leading to her defeat.

Benio's character is marked by a quick temper, a mischievous streak, and a fierce sense of pride. Despite her role as an antagonist, she is not purely villainous; her rivalry with Takuto evolves into a grudging respect. After losing her Cybody, she risks her life to restore it, driven by her obsession with becoming stronger, and challenges Takuto to a rematch. She uses her kendo skills, specifically her powerful first strike, but loses again because Takuto had learned to counter her technique during a practice match. This second defeat causes a shift in her perspective; she begins to acknowledge Takuto's genuine strength and develops a more amicable, though still competitive, relationship with him.

Her key relationships are central to her development. Her childhood friends, Honda George and Gouda Tetsuya, are fellow members of Filament, and the three share a history of working together to restore their families' honor. Her feelings for Sugata remain a constant, yet she shows visible jealousy upon hearing of his affections for others. Her dynamic with Takuto transforms from pure enmity to a more complex bond, with the two sharing friendly moments such as a barbecue, despite being on opposite sides. By the end of the series, the lost mark of her family, the "Pe" mark, finally returns to her. With her Cybody restored, she joins the final battle alongside Takuto and the other members of the Glittering Crux, demonstrating her ultimate commitment to fighting for her own ideals and her place as a true Star Driver.