How should I explain, Nana is Yaya. She appears everytime Yaya is in trouble or feels depressed and she does all the things Yaya wouldn't dare to do. She stands up against her enemy, she is very strong, tough and full of self confidece - really the opposite of Yaya.
Beacuse Nana is Yaya, she also has a crush on Moriyama-kun... and she shows it. At one of Moriyama_s concerts she steps onto the stange and kisses him in front of the whole crowd and tells him that she things he's very cool ^_^. Nana transforms back to Yaya whenever somthing "embarrassing" happens to her, e.g. she stumbles and hurts her head etc. The interesting thing about everything is that Yaya doesn't know anything about Nana and she always wonders what happened, when she gets back to her real self.
