Wonderweiss
Margela

Wonderweiss Margela is an Arrancar Aizen made for one purpose, and everything strange about him follows from it. He cannot speak beyond a few sounds, does not appear to understand what is happening around him, and behaves like a small child who has wandered onto a battlefield. He was built that way deliberately.

Aizen removed his intelligence, his reason and his ability to use Cero, and traded all of it for raw output and a single function. Wonderweiss exists to neutralise Genryūsai Yamamoto's Zanpakutō. When he releases, a second creature emerges from his body and swallows Ryūjin Jakka's flames whole, taking the strongest offensive Shikai in Soul Society off the board at the exact moment Aizen needs it gone.

Before that he impales Jūshirō Ukitake with his bare hand and catches Yamamoto's blade in his palm, which is not what anyone expects from him. Yamamoto kills him afterwards by punching a hole through his chest and letting the sealed flames out, and Wonderweiss, dying, reaches toward Tōsen's remains and makes a noise that is unmistakably grief. He was made empty and turned out not to be, which Aizen had not accounted for and does not comment on.