Aris is the kid who takes Teresa's spot in the dormitories at the beginning of the book. He's Teresa's Group B equivalent, as he was sent, comatose, to the Maze that was occupied by girls, just as Teresa was sent, comatose, to the Maze that was occupied by boys.
Aris doesn't talk much. In fact, he doesn't do much, either, but we can tell he's going to play some kind of significant role in the overall story. Not only does he have "a few splotchy memories"—meaning that he can remember a little bit of the past—but he's also even a figure in Thomas's dreams (16.26), which suggests that the two guys have some kind of connection in the past.
Aris plays the role of Teresa's best friend when the whole acting betrayal thing is going on, so for a while, it seems like Aris is kind of a bad dude. But after he stops playing the bad-guy role, and after Thomas dreams of him, we might just be inclined to think Aris is a good guy. Maybe even one of the key good guys.
We'll just have to read the next installment to find out.