The green-haired girl seems to be a new character from the "Chamber of Time" game named Verde Shidarisa.

I think I saw what this picture is referring to in the game. Verde sometimes is standing in the hallway in the dorm (she lives in the room between Akko's and Amanda's); if you make Akko talk to her, Akko asks Verde how she does her hair up like that and then Verde suggests that Akko try to curl hers up also.