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.