Karashi, also known as Mustard, originates from the Japanese folktale Kachi-kachi Yama, where she was the rabbit who avenged an old woman's death by killing a tanuki. She now serves as a torturer in Hell's animal cruelty section, specializing in punishing humans who abused animals during their lifetimes. Her appearance is deceptively cute, often leading others to underestimate her, but she exhibits extreme and ruthless behavior when provoked.

A defining aspect of her personality is an intense trigger response to the word "tanuki" or related concepts, causing her to snap into a violent, vengeful state rooted in her folktale backstory. This reaction stems from her deep-seated grudge against the tanuki, and she uses violence as a method to relieve stress. Despite her focused brutality during tortures, she displays a short attention span in daily tasks, often becoming easily distracted.

In her professional capacity, she collaborates with Momotaro's former companions—Shiro the dog, Kakisuke the monkey, and Rurio the pheasant—who also work in Animal Hell. Her efficiency and dedication to punishing animal abusers are acknowledged by Hozuki, who oversees her work. Outside her duties, she occasionally participates in Hell-wide events, such as sports days, though her focus wanes quickly during these activities.