A mysterious man in white clothes who hides secrets about Koro-sensei's past.

Shiro's real identity is Kotarou Yanagisawa, chief scientist of a project that involved trying to create antimatter without massive energy requirements by generating it in a living body. He is also the heir to a wealthy family and a genius biologist in charge of the world research foundation, but was also revealed to have no ethics or morals in regards his research. He himself admitted that he lacked the passion.

While still a university student, he met aspiring teacher Aguri Yukimura, whose family ran Yukimura Pharmaceuticals. As the company struggled to keep afloat, Yanagisawa bought the business and entered into an arranged relationship with Aguri. The real reason was that Aguri's passion for teaching would offset his lack of passion in his chosen field. Outwardly however, he stated that he only did so out of pity for her, and essentially treated her very poorly.
(Source: Ansatsu Kyoushitsu Wikia)