Greenberg never appears on screen. He is Coach Finstock's least favorite student. It is implied that he's a good student, at least in Economics, because it's common knowledge that he would have done the reading.
Greenberg is on the Cyclones lacrosse team.
It is possible that we saw Greenberg once in 1x06 - Heart Monitor. If Greenberg is a boy, which seems likely since Coach addresses the character in the boys locker room, then during a classroom scene in which Coach asks about the reading they were supposed to do last night, we see three boys raise their hands. Coach then tells Greenberg to put his hand down.
Only one of the three boys makes even the slightest motion to do so. This could be Greenberg. As you can see the other two boys clearly still have their hands raised, whereas the one in front is lowering his.