Serio, designated HMX-13, serves as the advanced technical successor and younger sister unit to the maid robot HMX-12 Multi. Her initial field testing occurs at Sanyo Girls' School alongside Ayaka Kurusugawa. There, she demonstrates capabilities surpassing Multi, including a satellite communication system for direct data transfer with the Kurusugawa mainframe host computer.

Unlike Multi, Serio initially exhibits no emotional expression and communicates using strictly formal Japanese, making her interactions feel less human despite her programmed core directive to assist humans. A fundamental limitation is her inability to comprehend complex human emotions, leading to failures in assistance. This emotional deficiency forms the central theme of the drama CD "Piece of Heart," where she struggles with nuanced human feelings and social contexts.

Her character development culminates in the anime sequel "Remember My Memories." After Multi is reactivated without memories, Serio participates in efforts to restore them. During this emotionally charged situation, Serio experiences a breakthrough: she sheds tears for Multi. This moment signifies emotional development despite her original design limitations. Her role proves critical to the resolution as she collaborates with others, including Serika Kurusugawa, employing unconventional methods to locate the missing Multi.

Visually, Serio features "dull eyes of unhappiness," lacking the reflective highlights seen in more emotionally expressive characters like Multi, reinforcing her initial emotional void. While her technical prowess remains consistent, her journey reflects a transition from a purely functional android to one capable of demonstrating genuine, albeit nascent, emotional connections.