Theo is a young human male with light blue hair and green eyes, characterized by naivety and inexperience. During a raid on his village, the demon Ziska discovers him, targeting him specifically due to his youth and perceived vulnerability. Overwhelmed and unable to resist, Theo lacks understanding of her actions.

When threatened by demonic presence, especially Ziska's advances, the spirit of a legendary hero awakens within Theo. This hero is the reincarnation of the figure who defeated the Demon King centuries prior. The transformation alters Theo physically into an adult male and shifts his personality to become assertive. In this form, he aggressively counters threats, using force to subdue demons like Ziska and neutralizing their abilities. This change is triggered solely by proximity to demons and recedes once the threat dissipates, returning Theo to his child form.

Though he reverts, the hero's spirit remains dormant within him. Between transformations, Theo retains his childlike innocence and naivety, making him susceptible to deception. Ziska repeatedly attempts to approach him in disguise, aiming to avoid alerting the hero's spirit. Theo often fails to recognize her initially, only identifying her after a mistake reactivates the hero's awareness.

Following his initial encounter and transformation, Theo chooses to pursue adventures. He maintains this resolve despite his ongoing vulnerability outside his heroic form. His journey features repeated interactions with Ziska, where his naivety and the hero's intermittent interventions create a cyclical dynamic of vulnerability and empowerment.

Theo's background lacks explicit detail beyond his village's destruction and the awakening of the hero within him. His development centers on the contrast between his inherent innocence and the inherited power that surfaces in response to external threats.