Sonya Sei is a member of the traveling musical group The Pussy Cats, a cover for her true profession as a professional assassin in the trio known as The Midnight Panthers. The group is managed by an elderly woman who coordinates their assassination missions.

She has dark hair, blood-red eyes, and a buxom figure. Her unique method of assassination utilizes her hair, which she can grow and retract at will. This hair is exceptionally resistant and functions like garrote wire, which she uses to strangle her targets.

Her operational methodology involves seducing male targets as part of the assassination process. She derives enjoyment from the psychological aspect of her work, specifically the feeling of superiority gained from exploiting her victims' unawareness of her true intentions. Both the act of seduction and the subsequent killing are sources of thrill for her, often resulting in a cold smile during executions.

She is a confident individual, fully aware of her seductive capabilities and believing that no man can resist her charms. This confidence is integral to her effectiveness as an assassin.

One notable mission was a contract to assassinate a despotic king. This operation held personal significance for another member of the group, though Sonya participated in it.