I'm the quiet, rational, somewhat deep and serious one that keeps everybody in check.
I'm usually the one that will start an interesting topic for discussion but may not say too much more except what is really important. I'm probably the most reliable out of the bunch as well as the most thoughtful so I'm often the one they call when they are in trouble.
I'm usually the one that my friends go to about their problems because I'm a great listener, with a lot of random knowledge and am never judgmental. I've barely said anything hurtful to or about anybody and wont let it happen around me. Though this sometimes can become tiring when it's about recurring relationship stuff (you know, when it's nothing but the same stuff for hours every day for weeks or months).
I'm also, unfortunately, the least social. I love my friends but they do stuff 3+ times per week (and I don't mean just chilling but actual events) and I've usually had my fill doing stuff once per week, so I'm sometimes dragged to places haha. Also when we go out they will be dancing and being super outgoing to all these new people. For me I'll dance sometimes and will strike up some conversations with new people or vague acquaintances a few times but I'm nothing like most of them.
This isn't to say I'm just some boring stiff though, I'm rarely a downer and I'm still fun to be around. I mean I drink, make jokes and do my fair share of stupid stuff too, just generally not as much as my other friends.
I'm also only one of two people who play video games more than once a month and I'm the only one that is kinda involved in geek/nerdy culture (though I'm far from the stereotypical overweight, neckbeard, dungeon master with no other interests, no offense meant). I have a lot of other activities that I share with my friends but sometimes it's kinda sad that I don't have anybody except online friends to share my second biggest interest with (after music/guitar).
I believe most groups have somebody like this though, so I feel I round out the group nicely.