Fighsly, originally called the Fighting Slime, attained a humanoid form through rigorous physical training. Her appearance includes long hair tied back, shown as blonde in the anime but with chin-length hair and a headband elsewhere. She wears a revealing white top resembling a gi jacket that exposes her shoulders and navel, along with white shorts and red knee-high combat boots.

Driven by a misunderstanding of human monetary systems and the perceived importance of currency, Fighsly seeks prize money through martial arts tournaments. This opportunistic nature and lack of integrity led her to abruptly switch allegiance from seeking Azusa's mentorship to pleading with Beelzebub instead, after sensing Azusa's dark aura—a residual effect of centuries of slime-slaying—which unsettled her.

Fighsly possesses specialized knowledge key to resolving Falfa's accidental transformation into a slime. When Azusa requests her help, Fighsly initially refuses, resulting in a confrontation. Azusa must then defeat her in a martial arts tournament without magic to secure her cooperation, demonstrating Fighsly's martial prowess.

Beyond tournaments, Fighsly engages in exploratory adventures, such as accompanying Azusa and Beelzebub on expeditions into ancient ruins. These outings, often encountering complications, show her integration into broader storylines and a willingness to participate in diverse endeavors that challenge her combat skills or align with her interests.