Chomusuke is a black cat with a red cross-shaped marking on her forehead and bat like wings. She has stubby legs, a big round head, and yellow oval eyes. When she's happy or angry, she shows sharp teeth.
Chomusuke cannot speak human language, but she seems to be fairly intelligent and can understand human speech to some extent. One time, Kazuma saw her heating up leftover fish with fire breath. Her true power was sealed some time ago, thus leading Chomusuke to being the target of bullying from other kids.
She is friendly with almost everyone except Aqua and Emperor Zell. She really likes Wiz and Kazuma since they treated her with extra kindness.
Ever since she absorbed Wolbach, Chomusuke has begun taking up more of her personality traits. For example, Chomusuke has begun refusing to eat "pet food" such as leftovers or raw fish, instead preferring "human food" like biscuits. She has also started liking baths just like Wolbach did, even though she previously hated them, and even chooses to bathe with Kazuma just like Wolbach did. Furthermore, she seems attracted to traits like sloth and violence as she is shown to enjoy Kazuma's laziness and Megumin's aggressiveness.
Long ago, the evil Goddess of Sloth and Violence, Wolbach, was defeated and split into two parts, which were then imprisoned within a complex sealing stone. One of the entities was Wolbach, who represented the aspect of Sloth; and the other one was the creature that would become Chomusuke, a violent black beast that represented Violence. The sealing stone was eventually taken to the Crimson Demon Village as the Crimson Demon Clan wanted to use it as a tourist attraction.
Around 8 years before the main series, a young Megumin played with the complex sealing stone as a puzzle toy, and accidentally released Wolbach and Chomusuke. However, as soon as they were released, Chomusuke, while in her true form, a giant black beast, went out of control and tried to attack Megumin. In order to protect her, Wolbach used Explosion to defeat the beast and saved Megumin. She then drained the beast of most of its power and size, turning it into a small cat. After doing so, Wolbach also resealed Chomusuke in order to be released at a later time.
Seven years later, Komekko, Megumin's much younger sister, who didn't have any proper toys either, also solved the sealing stone easily and unsealed the black cat again. In her weakened state though, the cat was no match against Komekko and almost became a family meal on several occasions before being adopted as Megumin's familiar.
- While coming up with a name for Chomusuke, Yunyun came up with the name Kuro (which just means black). Megumin thought the name was weird and decided that it will be a temporary name for Chomusuke.
- Kazuma is among the names that Megumin's classmates recommended for Chomusuke when Megumin deemed the cat to be her familiar.