Well, overall, not much is known about Jin. He isn't exactly a huge character. Just a brief side-character. He does happen to be one of the more popular and solid characters introduced in The Dark Tournament. He (and Touya, surprisingly enough) were big enough characters to reappear! (gasp) Most of what I can tell is theory, but it makes sense, nonetheless.

But what is known about Jin is that he is a Yokai of the wind and is also a stealthy ninja well-known in the Makai. Whether he is regarded as an enemy is hard to say for sure since, again, the show/manga never delve into his past, but he seems to want the best for himself. He doesn't want to hurt anyone.

During The Dark Tournament and his battle with Yusuke, Jin flies up into the sky and just sits admiring the world around him. In that moment, I believe you see the part of him that is his purest part. The part that just wants to be free and feel like there's nothing burdening him. Aside from the fact that he is participating in a battle tournament (and considering the life of a Yokai has to be significantly different than that of a human's), he must have had a pretty relaxed and at-ease childhood. He doesn't seem to hold any grudges or have a problem opening up to people.

Jin is very happy-go-lucky, silly, and fun-loving. He is clearly always up for a good time and doesn't seem to take too much seriously. When he needs to get serious, he does, but he seems to enjoy romping around more. Such as in Makai when he is eavesdropping on Yusuke's meeting with Yomi--he is the first out of Kurama's group to jump out and greet Yusuke with a rub on the head and a hug. ^.^

From all this, I think we can conclude that Jin is fighting with Team Mashotsukai primarily because they have a chance at winning the island and "freedom". Jin is too smart and good a person to actually like some of the guys on that team, so I'll bet he's just sticking around so he can get that island and reach his goal of a little peace of mind.

Jin sticks close to his good friend, Touya. Touya is kind, loyal, and good-natured, so it's no surprise that out of Team Mashotsukai, they're the ones who stay friends.

Anyway, that's as deep as I can go into Jin for now. Again, he is not a private character, per se, but just one with such little screentime. We all love him, nevertheless.