How to get Nana in your party
To get Nana,you must defeat Mission 5 (Nordillian Assault). Then when Bastion and his crew discuss the matter at the Castle, a messenger from the Muspel Nation will arrive and ask Bastion to meet with Duke Radcot. Two options will appear:
1. Actually , Zeira, I'll meet with him, and etc.
2. "Can't remember the text", but it don't matter.
Pick the first one. Then you'll be given another choice,choose: Yes. Tell him that you will depart soon. Now your onto M6, during the battle let Bastion be deafeted and you will be held captive. Afterwile Nana arrives and she tells you that she has secret passage way out of the fortress. After some chit-chat your onto Mission9 : Desert Escape. Beat this battle, and the next and the next assault. Then when after Mission 13: Kaidul's Revolt, Galvas will ask you about helping Thomson, Bastion will agree. Finally a messanger will come and say the scouts have sighted the Bahamut ATAC and it's headed for the castle. Guess who's inside? Right nana. Now you have control of a power ATAC, Nana is at Level 6, so you can mold her into whatever you want.
