Sena Mizuochi is a soldier of the Winfield Kingdom, assigned as the primary protector of Crown Prince Kurisu Ousaka. He maintains cover as a biology teacher at Yuusei Academy while also serving as the shooting club's counselor. Following their escape from Winfield, Sena continues safeguarding Kurisu alongside fellow guardian Shion Toudou.

Physically, Sena is a tall adult in his mid-to-late twenties, notable for his red hair and blue eyes. He is often depicted carrying a pistol and occasionally wears glasses. A blue bird named Pipi is frequently seen accompanying him.

Personality-wise, Sena exhibits a stoic and reserved exterior, initially appearing cold. This stems from difficulty expressing emotions rather than lack of feeling. Underneath this demeanor lies strong loyalty and protectiveness, especially towards those in his charge. His working relationship with Shion Toudou remains professional, focused on their shared duty to Kurisu.

Sena's background includes a pivotal role during the attack that killed the Hamura parents; he rescued the infant twin princes, Shou and Kai, forging his long-standing connection to royal affairs. His character holds indications of a past crush on Mari, the twins' late mother. Across documented appearances, his role consistently centers on protection, duty, and preserving his academic cover, with interactions reflecting his reserved yet unwavering commitment.