Theresia is a Lizardman demi-human and the first seeking companion of the protagonist, Arihito Atobe. Her background is that she originally died as a Seeker within a Labyrinth and was reincarnated or transformed into her current demi-human form. Due to the lateness of Arihito's visit to the Mercenary Office, Theresia was the only Lizardman mercenary still available for hire. Impressed by her efforts on his behalf, Arihito purchased her contract for one hundred Mercenary tickets, equivalent to thirty gold, making her a permanent member of his party.

In terms of appearance, Theresia looks like an athletic young woman with only a few traces of her demi-human nature, including a tail, scales, and a cold-blooded constitution. She typically wears a hood that covers her long black hair, and there is a magical slave contract seal on the back of her neck. Her mask, a feature of her Lizardman heritage, is not removable. Her personality is characterized by a strong sense of loyalty and a stubborn, steel will. She is described as a highly skilled Rogue and a most trusted companion. She is not very talkative, often communicating with silent nods or actions. Theresia is also shown to be appreciative, once attempting to express her gratitude to Arihito in a bold manner, though her actions can be influenced by her physical needs.

A key aspect of her personality is tied to her physiology. As a cold-blooded creature, she is vulnerable to heat; a hot bath will leave her weak and unable to move. Despite knowing this, she possesses a stubborn determination and wishes to bathe with Arihito regardless, which he finds both healthy and perplexing. Her motivations are deeply tied to her service to Arihito. She was purchased as his companion and is fiercely dedicated to him, to the point of refusing to sleep so she can keep watch over him, though she inevitably succumbs to exhaustion. Arihito also holds a personal goal to bring Theresia to the Cathedral in District Four to return her to her human state.

Theresia plays the role of a Rogue or midguard in the party, using her skills to scout, detect traps, and launch surprise attacks. Her role in the story becomes critical when she is inflicted with a curse by the Shining Simian Lord. This curse progressively destroys her and will eventually turn her into one of the Simian Lord's mindless underlings, creating a time-sensitive rescue mission for Arihito and the party. Her key relationship is with Arihito, her master and party leader, to whom she shows unwavering loyalty. She also interacts with the other members of the party, including Kyouka Igarashi, Suzuna Shiromiya, and Elitia Centrale. Theresia's development is tied to her growth as a Seeker and her struggle against the curse. She gains levels and acquires new skills as the story progresses, evolving from a simple hired mercenary to an integral and cherished member of the group. Arihito is also mindful of her needs, such as searching for heat-resistant equipment to help her overcome her weakness to hot baths.

Theresia possesses a number of notable abilities, primarily focused on stealth, scouting, and combat. Her initial skills include Accel Dash for quick movement, Expansion of Search Enemy to detect foes, Silent Walking to move without making a sound, and Warning to notice surprise attacks. As a demi-human, she has the Lizard Skin skill, which improves her defense and attribute resistance. Her combat skills include Double Throw, Reverse Grip to increase critical chance, and Sneak Attack, which doubles damage when attacking an unaware enemy. She is also proficient in trap-related skills, such as Trap Detection and Activate Trap. Her Morale Discharge skill is Triple Steal, which gives the entire party a chance to steal vitality, magic, and dropped items from a monster. She can also use weapon skills, such as Wind Slash, performed with a sword that has wind-type abilities.