That's because she's mini-Tatsuta, she smiles even when angry or frightened. If the smile ever goes away, it means she's .2 seconds from disemboweling you.
Nagato-san became Kai-2.Go and tell the Admiral that...Don't wanna!!I'm gonna be a transport shiiip!!I was the flagship and everything... why did they all run away from me at flank speed...!!Nagato, an order's come in to sortie to E0...Heeeey!Come over heeeere!Don't you worry! I won't let even a single shell touch the destroyers!You can't go touching either!Wanting to go on expeditions with destroyers... that's all kinds of terminal there...
