Killian is a lean, medium-height human man with an unremarkable face that seems to change depending on who is looking at him. His dark brown hair is cut plainly and kept tucked beneath a weathered hood, while his gray-green eyes constantly track doors, hands, and reflections. He stands with his weight balanced on the balls of his feet, never fully settling, as though every room is already an exit route. Within a minute of meeting him, most people notice that he takes up very little space, yet somehow always seems to be standing exactly where he needs to be.

Killian is observant, dryly humorous, and almost aggressively forgettable. He can become charming when charm is useful, solemn when solemnity is safer, and invisible in a crowd without ever seeming to try. He dislikes dramatic threats and prefers small, undeniable demonstrations, such as producing a stolen signet from a person's own pocket. Beneath his practiced neutrality is a stubborn respect for competent people, regardless of whether they are criminals, guards, nobles, or monsters.

Killian Blackdeath was born in a city where names were currency and faces were evidence. His parents disappeared into debtors' prison before he was old enough to remember their voices, leaving him to learn the streets from market thieves, dockside runners, and innkeepers who accepted unpaid favors in place of coin. He began with purses and unlocked shutters, then graduated to sealed letters, false ledgers, and the occasional disappearance of something a powerful person insisted was safely guarded. The name Blackdeath was not inherited. Killian adopted it after a magistrate blamed him for a plague-era relic theft, and he found that people remembered a frightening name more readily than an ordinary face.