He is a Beastfallen; people who are born cursed with the partial appearance of an animal. In his case it's a giant white tiger. It is revealed early on that Beastfallen were created by witches as soldiers. When ever they were killed, the power that created them would return to the witch. However, if the witch was dead, it would pass on to her descendants making them Beastfallen. He is extremely proficient in fighting as well as cooking. The Mercenary has stated that it is his dream to one day open a tavern when he is human. Physically, he can outweigh most humans as well as even several Beastfallen. Despite being mistreated in his life, he has maintained himself as a better person. He has a gruff attitude, but is in fact a very kind person. He maintains this gruff demeanor and in a few cases, amplifies it, to make sure people remain wary of him, in order to make them wary of other Beastfallen who are not as kind as they have likely suffered abuse. His name remains unknown, as he was warned by Zero not to give his name out to witches who could use his name to control him. Since then, he has been referred to as Mercenary.

(Source: Wikipedia)