Daisy Dursley knows a lot of things, like how to fold your socks, long division, that flying was a death wish, and that her baby brother was a complete menace. The most important thing that Daisy Dursley knew, however, was that she would NEVER understand the insolent git that was Teddy Lupin.
Teddy Lupin also knew a lot of things, but he keeps that to himself, can't spoil the reputation after all. One thing he won't keep to himself, however, is that Daisy Dursley will NEVER change from being the vapid little girl he met on the train.
Of course, neither of them knew anything, they'd learn that eventually.