In Isekai Quartet 3, Chomusuke is a small black cat who attends school as a member of Class 2, having been introduced as a transfer student. While her classmates come from various otherworldly backgrounds, Chomusuke stands out as an animal, though her presence is quickly accepted by the others as a relatively normal addition to a class that already includes demons and undead beings. She is easily identified by the small bat-like wings on her back and a distinctive red, cross-shaped marking on her forehead.

Chomusuke is the familiar and constant companion of Megumin, the young archwizard from the Crimson Demon Village. Her background is far more extraordinary than her cute appearance suggests. She is actually the embodiment of the violent half of the dark goddess Wolbach, a deity representing Sloth and Violence. Long ago, she was sealed away, but was accidentally released by a young Megumin. In her true form, Chomusuke was a giant, destructive black beast, but she was defeated by Wolbach, who drained most of her power and compressed her into the small feline form she retains. She was later released again by Megumin's sister, Komekko, who intended to cook her for a meal, only for Megumin to rescue her and officially adopt her as her familiar, giving her the name Chomusuke.

In terms of personality, Chomusuke is generally docile, affectionate, and calm, showing a particular fondness for Megumin, who she rarely leaves. She is also very attached to other members of Kazuma's party, including the lazy adventurer Kazuma Satou, whose slothful nature seems to attract her, and the crusader Darkness. However, she displays an intense and unexplained animosity toward the goddess Aqua, whom she often hisses at, scratches, or actively avoids. This aversion is believed to stem from a fundamental opposition between Aqua's divine, holy aura and Chomusuke's own origins as a demonic or dark entity.

Throughout the story of Isekai Quartet, her role is primarily that of a quirky mascot and a member of the student body. She participates in school activities like any other student, such as being assigned to a group for a flower-growing project alongside Subaru, Beatrice, Mare, Cocytus, and Rhiner. During these activities, she serves as a quiet observer, highlighting the absurdity of the situations the characters find themselves in. Her presence also creates jokes centered on the contrast between her extraordinary nature and the insistence by Megumin and others that she is just a normal cat.

Despite Megumin's insistence that she is an ordinary feline, Chomusuke possesses several notable abilities. She can breathe fire, a skill she learned from the Axis Priest Cecily. She is also capable of flight using her small wings. These abilities are often displayed in front of the other students, much to their surprise, though Megumin typically dismisses them as figments of the imagination. Her true nature as a fragment of a dark goddess also makes her a target for various demons who seek to capture her and reclaim her power.

Throughout Isekai Quartet 3, Chomusuke does not undergo significant personal development, instead remaining a static but beloved fixture of the class. Her key relationships are an extension of those from her home series: unwavering loyalty to Megumin, a close affinity for Kazuma, and a comedic rivalry with Aqua. Her integration into the school, being treated as a student, is a recurring source of humor that parodies the conventions of the crossover setting. Her development as a character is largely historical, rooted in her past as a sealed beast and her transformation into Megumin's pet, a process that has already concluded before the events of the series.