Gertrude is a man-made demon created by Claude, assembled from select parts of demons that were either stolen or found. Since none of his body parts are originally his, he spends his life evading various demons who seek to reclaim their respective pieces. Gertrude seeks the recipe for his own creation in order to stop similar cases from happening, to understand himself better, and to deter demons from pursuing him.
His power significantly increases during full moons, but he tends to avoid leveraging this boost as it makes him more aggressive. During these periods, his normally silver hair turns black due to an "evil aura." Puppen hints that Gertrude occasionally befriends humans, but these friendships tend not to last. Sahara's friendship marks the first time he has maintained a connection for such an extended period and stayed in one place.
Gertrude possesses the ability to exert control over anything by writing "Frailty, thy name is..." with his own blood.
