Who is the green ninja Ninjago?

Answer 1

If you look closely at the faces (more specifically the eyebrows) of all four ninjas, and then that of the green ninja, you will see that one of their faces match up exactly with the green ninja. That ninja would be Kai, Ninja of Fire. This is also supported by the fact that Kai is the only ninja that has used a new technique with his golden weapon; thus being farther along in training than the other ninjas.

Answer 2

No one really knows. Watch the episode on March 21st to find out!