Rinko Sakuma works part-time as secretary and assistant at Akutabe's detective agency. She initially joined as a university student but left voluntarily to focus on breaking a personal curse. Her appearance features shoulder-length brown hair, green eyes, and distinctive half-moon red-framed glasses. She typically dons casual attire like a loose pink cardigan with jeans and sneakers, frequently changing outfits across the series.

Her personality blends inherent kindness and shyness with a pronounced violent, sadistic streak when angered or frustrated, often resulting in physical outbursts. She possesses a strong sense of justice but demonstrates a recurring weakness for financial gain or short-term rewards, influencing her decisions during investigations. Alcohol intolerance exacerbates her aggressive tendencies, causing drastic behavioral shifts when intoxicated, including bullying and heightened assertiveness.

As a demon contractor, she exhibits significant talent in summoning and commanding demons, initially bound to Azazel and later forming contracts with Beelzebub and Incubus. Under Akutabe's mentorship, her magical abilities expand considerably, progressing from basic interactions to executing complex spells capable of physically harming or destroying demons. Her intelligence facilitates rapid learning of occult knowledge and incantations, with demons noting her increasing resemblance to Akutabe in power and demeanor.

A critical aspect of her background is a curse that inflicts disaster or death on any man developing romantic or sexual feelings toward her, triggered by physical contact like a kiss. This curse motivated her university departure and fuels her ongoing quest to identify and confront its demonic source.

Her professional relationship with Akutabe blends respect and intimidation, viewing him as a mentor despite his ruthless methods. She maintains strained dynamics with her contracted demons: frequently cold and distrustful toward Azazel due to his perversity, respectful yet disgusted by Beelzebub's fecal-related habits, and strict with later contractors like Incubus. She initially dislikes Kotaro Dochin but gradually develops a coexistent rapport.

Character development shows her evolution from a naive novice to a confident demon detective permitted to handle cases independently. Temporary removal of her glasses via Undine's power reveals hidden truths but causes distress, reinforcing her reliance on them. Involvement in a university cosplay club, where she adopts a "Strawberry Warrior" persona, briefly diverges from her detective work but demonstrates her capacity for role immersion.