LogFAQs > #910368023

It's less 'too shy to admit it' and more 'denies it/treats them like they don't like them but actually does'

Undyne is actually a pretty good example of a tsundere toward Papyrus (though of the friend variety and not lovers). Dumps on him all the time, but actually cares about him a lot, especially when he's not around.

Probably the most stereotypical tsundere line is after they help out the one they're tsundere towards/give them a gift/something like that, they say something to the effect of (lets say they give them cookies as a present) "Don't get me wrong, I didn't do it for your sake or anything! I just made too many and didn't want them to go to waste!"