Serio HMX-13 is a supporting character from the anime and visual novel series 'To Heart.' She is an advanced humanoid maid robot, designed as part of the HMX series and serves as a prototype model. Unlike her 'sister' Multi (HMX-12), Serio is calm, stoic, and emotionless, often struggling to understand human feelings. Her primary role is to perform maid duties and assist the main characters, providing a contrast to Multi’s cheerful personality. Serio’s development explores themes of artificial intelligence and emotional growth.