Woohyuk was born (more like made) in a lab as an experimental subject.  Due to a genetic mutation, he has no tail.  Ever since he was born, he's lived in a small laboratory inside a locked room and was never allowed out.  All he learned to do was to get around the rules by giving the people who work at lab benches the relief both physically and mentally, even if it meant stress to him.  He's always put people's needs before his, even through his heat cycles, learning to cope with it for years.  In the course of his 20 years at the lab and due to him putting other's needs before his own, he's been through quite a lot of different extreme situations when it comes to ual .  Apart from the minor ones on his back, it may seem as if he doesn't have visible scars (well he was at a lab all his life, scientists, nurses and doctors were always readily at hand).

When he was 21, he escaped the lab and was taken in by an old couple who owned a massage shop, letting him work there to afford for himself.  The old couple handed the parlor over to him, making him the manager of the shop even at an early age of 24.

HIs hyperuality comes from a mistake in modification while he was 'being made'.  One of the scientists working on this 'project' he accidentally put in a gene that encodes for over production of testosterone, leaving him in a constant needy state.  

He also has a high, very high pain tolerance he's acquired through his life at the lab.  after all, he was an experimental subject being tested on constantly (physically and mentally)

broken. not looking. i don't want nor need your pity.