Clementine

Clementine is a major antagonist in the early arcs of Overlord.

She's a former member of the Black Scripture (one of the strongest special units of the Slane Theocracy) who became a chaotic evil serial killer after deserting.

Key traits:

• Extremely sadistic and bloodthirsty
• Takes great pleasure in torturing and killing her victims in creative/humiliating ways
• Highly skilled assassin and combatant, specializing in dual-wielding stilettos
• Superhuman speed and agility, even among the New World's elite
• Wears a distinctive outfit with a red scarf, short skirt, and a creepy perpetual grin

She appears mainly in Volumes 2 and 3 (and the corresponding anime Season 1 episodes), where she becomes one of the first human characters to seriously underestimate Ainz Ooal Gown — with very predictable (and gruesome) results.