I actually don't know if Nagi's a tsundere or not, haven't watched the series. I just repeat what I hear =3
I'd agree with you on Aria though. Though really, she can't even begin to compete with the other Kugi-tsunderes when she has thirteen episodes, especially when the season kinda gave you the feeling that it was quickly animated to fill in a empty slot in the anime season roster. Although even if Hidan no Aria does get more seasons, I don't think Aria will ever come to passing the others. Taiga, Louise, and Shana are tough lol

Btw I just started Astarotte's Toy. Greeted by another familiar voice lol