Saerin is a sarcastic, witty loner who pretends to be content with his solitude. Ever since he was little, he'd pretend he didn't need anyone in his life. That, of course, was a coping mechanism to get over his father leaving him before he was even born. If he didn't need anyone, he certainly didn't need that man. The sarcasm and wit is just a good way to hide the pain.
As tough as he may try to come across, Rin is a big softie. He lives by his own set of rules, sure, but those rules are usually heartfelt. He has a real soft spot for kids, and wants to adopt a few when he gets older. He can't see himself getting married (we'll see about that), but he knows he wants some kids running around in the future. He also gets easily hurt. Rin's really good at hiding it, but things get to him rather easily. His mom says he's too good at hiding it, and that those bottled feelings will just burst one day.
Because of everything with his father, Rin is not the most trusting person. He doesn't get real close with people. When he does, they should feel honored because they've earned his respect in some form or fashion. More so because of his mom, he's a hard worker and gets things done. He just likes going about things in a more proficient way. Work smarter, not harder.
Saerin is also quite the charmer. He doesn't mean to be, but he has a sort of dark and alluring vibe to him. Maybe it's learning to read people over the years, but he's just easy to have a conversation with. That comes in handy when you're doing a tattoo for a couple hours.
Speaking of, Rin has a special gift for tattoos. He's been fascinated by them since he was a kid, and never lost that fire. He likes testing the limits of magical tattoos and inventing new sorts of inks that can do different things. He's always been quite the inventor, but only on the down low.
Kim Sunhee and He-Who-Shall-Not-Be-Named (no not Voldemort, just an ass) fell in love early in life before either one knew what they were doing. Sunhee was only seventeen when she got pregnant. She was eighteen when she actually delivered the baby boy. Of course, the two weren't married so at the first sign of a child, the dipshit dipped. Sunhee's parents had both passed away when she was a girl, so she had no one to fall back on. She was pretty much alone in raising the baby.
That baby was named Kim Saerin after a kind man who helped Sunhee get back on her feet. She was able to get a job, a decent apartment, and a good friend. That was all she needed. Sunhee worked hard to give Rin everything he ever needed. It was tough, and growing up, Rin went to bed with an empty stomach more often than not. But it was still good. Small, broken - but still good.
At age seven, Saerin showed his first sign of magic. He wanted to go play with some kids in the apartment complex, but Sunhee demanded that he finish putting the dishes away first. Suddenly the plates and silverware were making their way to the cabinets and drawers without Saerin moving a finger. He only dropped three dishes and broke one! Pretty impressive. Sunhee didn't get a chance to talk to him about it though because he'd already run outside to play.
That same year, Rin went off to Mahoutokoro. There, he made friends with the "weird" kids, or outcasts. He could've easily fit in with the cool kids, but he was right where he wanted to be. He got his first tattoo his fifth year and was absolutely mesmerized by the art. He knew from then on that's what he wanted to do with his life. Rin gave his first tattoo the following year.
Come graduation time, Saerin had drifted away from most of his friends and was just ready for his own tattoo shop. He started looking for any places that would hire someone so "young" and "inexperienced" as he was. He found gigs here and there while he saved up for his own parlor. It's to the point now where he could almost open up. Now it was just a matter of finding a place.