What bane for Nino?

Def, without a shadow of a doubt. She can die to Reinhardt with an HP bane, and even then her defence is bad enough that you can dump it - she's gonna likely get one-shotted by DC units as is, and you should have someone else to take care of them anyway.

Def is her best overall bane, and I believe Res is actually her best bane at +10 since she can actually survive certain DC users without a Defence bane whilst still being perfectly proficient at handling Reinhardt despite the Resistance bane.

-HP would take away from both her physical and magical bulk which is a big no no.

I agree with these guys, think of it like this - at Nino's speed tier, she isn't going to be getting doubled. that means that, in the space of the ONE hit she will be taking, HP is your most valuable stat, since it counts as 3 extra points towards surviving physical OR magical attacks. this is excluding brave weapon users, since they hit twice. But, she won't be surviving physical brave weapon users of any kind regardless. So, -Def is your best bet. If/when weapon refinery for the Blade tomes comes out, and they give her an HP boost, then perhaps HP will be her best bane, who can say. but for now, Def it is I think

I run a +spd -hp Nino+10 with summoner support. She's most useful on an offensive arena team instead of defense. I have baited Reinhardts and a few Brave Lyns with Nino thanks to the atk ploy 3 seal and is a huge reason why I would think -res bane is Nino's worst out of -hp, -def, and -res. The +5 hp from summoner support patches up a -hp Nino much easier IMO, but if you don't give her summoner support, -def would be better to tank moonbow Reinhardt.