Sebastian, chief butler of the Yotsuba family, pledges unwavering loyalty to Alice Yotsuba, transcending traditional duties by actively aiding the team’s missions. He employs advanced technology such as the Yotsuba satellite radar to track Royal Crystals and meticulously coordinates mission logistics. Trusted with the team’s secret identities, he rigorously safeguards their activities from public exposure.

A pivotal moment arises when he engineers the Artificial Commune, a device replicating the Lovely Commune’s function, enabling his temporary transformation into the superhero Cure Sebastian. This guise grants superhuman speed and the "Sebastian Attack," a seismic strike maneuver. The device later falls into Marmo’s possession, triggering her metamorphosis into Cutie Madame and instigating a confrontation.

Having served Alice since her childhood, Sebastian shares a familial bond with her, often offering paternal guidance and emotional steadiness. During the clash with Cutie Madame, he dons knight armor to retrieve the Artificial Commune but is overpowered. Alice ultimately resolves the conflict, prompting Sebastian to voice pride in her maturity and vow to support her without artificial enhancements.

His brief superhero persona subtly parodies Batman’s aesthetic, and his non-canonical role as the first male figure to adopt a Cure-like identity underscores his unique position within the team’s support structure. His blend of technical expertise and heartfelt devotion cements his role as both strategist and emotional anchor.