I had to look it up, but Nicole is the mascot for Bump of Chicken, sort of like Eddie is to Iron Maiden. There's a surprising amount of merch of her online.
Did the TTK really chose the Soviet V Day as passcode?
They might be able to choose them themselves since it would normally be the Admiral and Nagato's passcode that would be used, presumably they don't want to hand those out (and then need to change them) every time they have to go off base.