Aiko Mary Harmony is a fifteen-year-old girl who serves as a central figure among the young protagonists of Gad Guard. She is the adopted daughter of Larry Harmony, a powerful businessman who owns Central Electronics, the largest electric company in Unit Blue. Because of her father’s status, Aiko lives in the affluent district of Gold Town, enjoying every luxury. Despite this privileged upbringing, she is not portrayed as spoiled. Instead, she is a sweet, loving, and deeply conflicted individual, constantly feeling the pressure to one day succeed her father in running his vast corporation, a future she is uncertain she wants for herself.

Aiko’s defining trait is her powerful idealism. She holds an unshakable belief that because all five of the main children in the story were given Techodes, they share a profound common bond and therefore should all be friends. This conviction drives her to persistently reach out to the others, even when met with disagreement or hostility. Her determination is resilient, as she believes there is good in every person and that everyone deserves a chance. In the narrative, Aiko acts as a crucial bridge between the other four key characters. She offers friendship to Arashi, provides reassurance to Hajiki and Takumi, and serves as a peacemaker to the more antagonistic Katana.

Her most significant relationships are with Katana and Takumi Kisaragi, the latter being the operator of a Techode named Thunderbolt. Aiko grows attached to Takumi after recognizing him despite his lack of a costume, and she becomes a persistent ray of sunshine in his life, cooking for him, keeping him company during bouts of depression, and working tirelessly to cheer him up regardless of his bitter attitude. Her connection to Katana is built on a foundation of trust and persistent goodwill; she visits him and his companion Sayuri in their hideout, bringing lunches and snacks to demonstrate her concern and good faith. Her friendliness is such that even an antagonist like Jacque Bruno cannot help but like her determination, particularly after she saves Sayuri from being crushed. In a notable event, she even invites Katana to a party at her father's mansion, an idea the other characters consider insane given Katana's nature.

Aiko does not pilot a Techode or possess any notable supernatural combat abilities herself. Her role is defined by her emotional strength and her function as a social and moral center for the group. Her development is seen in her unwavering commitment to her ideals in the face of a cynical world, using her position and her persistent kindness to forge connections between characters who would otherwise remain isolated.