Sylveon's ability Cute Charm activates when a Pokémon of the opposite gender uses a contact move on it. This results in the opposing Pokémon becoming Infatuated with Sylveon. It's quite situational and not that useful. Sylveon's other ability is Pixilate, which turns Normal-type moves into Fairy-type moves (and gives it a x1.3 boost), which is more useful.
While Sylveon's Defense is quite low, it makes up for it with a great Sp. Defence stat. It's Speed is also quite low at 60, which is something to keep in mind because many Pokémon will be faster than it.
Fairy-type Pokémon are weak against Poison and Steel-type moves. But they are immune to Dragon-type moves, while being super effective against Dragon, Fighting and Dark-types. Fairy-type Pokémon are also the second rarest type after Ghost-type, with only 4.8% being Fairy-type. The lower number of Fairy-type isn't too surprising, though, considering it was only introduced in Pokémon X/Y.
Sylveon's movelist from Pokémon X/Y.