He is of mid height, he wears a broken mask with stitches and a ragged cloak. He has a messy black hair, under his cloak, a worn out haori can be seen, based on its design, it can be easily said that he wears a Captain's haori, although the squad symbol cannot be seen for something or someone had pierced him from his back. He also has the number zero on his left chest, and a question mark forming form his mask.
He talks in a very scary voice, he even makes creep gestures like spinning his head and twisting his limbs, he also hates Aizen for many reasons, one, Aizen took his position of captain, two, he was made into a monster, and lastly, he was forced to leave Seireitei and go to hiding.
When he was young he was a kid genius, some even says that he might be the next captain of his squad, the 5th Squad under the time of Shinji, he had a rival that he considered his closest friend, Aizen, then when Aizen framed Kisuke, the 12th Squad captain of that time for conducting illegal researches, he was the number one candidate for being Captain, he was Captain for about two and a half years, but later on Aizen, betrayed him, he was experimented upon and was injected several genes of different hollows, as a result, he became a monster. Before Aizen left Seireitei, he escaped and went to hiding seeking revenge for what Aizen did to him, then he uses the name, Unknown.