Felix is a lean, wiry tiefling with soot-red skin, short black horns, a narrow face, and a long tail marked by old burn scars. Pale surgical scars cross his neck, ribs, wrists, and left temple, while several small circular scars dot his arms where restraints and instruments once held him. His yellow eyes are constantly alert. He dresses in worn leather armor beneath dark clothing, keeps his hood raised whenever possible, and carries his weapons with practiced economy.

Felix is watchful, dryly sarcastic, and fiercely independent. He rarely gives a direct answer when a half-answer will protect him, and he studies people as if searching for hidden instruments beneath their clothes. Despite his distrust, he has a quiet tenderness toward frightened, injured, or imprisoned people. He understands pain intimately and will often risk himself to give someone else the chance to escape.

Felix was born in a city where his infernal blood made him an easy target for suspicion, but prejudice was only the beginning of his suffering. As a young man, he was abducted by a private physician who claimed to be studying the relationship between fiendish heritage and physical resilience. Felix endured years of experiments involving bloodletting, alchemical injections, surgical alterations, and magical tests. He survived, but the procedures left him covered in scars and taught him that every smile could conceal a knife. During his escape, he stole the physician's keys, picked his first lock, and used a thin surgical blade to cut through the only chain he could reach.