Wonderweiss Margera is an arrancar in Sōsuke Aizen's army. Though not an Espada, Soul Society ranks his spiritual power in the same class as them. Wonderweiss is small, thin, and somewhat child-like. What is left of his hollow mask remains on top of his head, and resembles a tiara or small crown. Wonderweiss's personality is also child-like and even autistic. When sent on a mission to kill, he instead plays around with a dragonfly. He shows a bit of hostility, firing a retaliatory bala blast at one point, but quickly reverts back to his regular state after doing so. He isn't able to speak clearly, only saying a few syllables at a time like a child would. Kaname Tōsen says that Wonderweiss, like himself, is a pure being (though he makes a point of wondering whether he is pure good or evil), which often tend to stick together, causing Wonderweiss to follow him everywhere when possible. Also for this reason, he tried to restrain Gin Ichimaru from getting closer to Tōsen, since Wonderweiss was wary of him.

Wonderweiss's zanpakutō is a large purple sword strapped to his back, about as tall as he is. The hilt resembles a western long sword rather than the more common katana, although the familiar hilt weaving is present, whilst the guard resembles an Egyptian eye. The name and released state of his zanpakutō have not been revealed.

It is revealed in the "Fake Karakura Town" arc that Wonderweiss is a so-called "modified arancar", and that he was specifically created for the sole function of sealing Ryuujin Jakka's flames. Aizen considered Head Captain Yamamoto to be the only real threat to his plans, so he created Wonderweiss by sacrificing all of the arrancar's emotions, memories, and ability to speak.