remix if you would take this Sylveon in after its trainer left it. Who in the name of the moon would abandon a Sylveon just because it didn't evolve into an Umbreon? And also, according to Pokemon logic, you're supposed to have a high friendship with your Eevee and evolve it at night in order for it to become an Umbreon. PLUS, Sylveon is as strong, if not stronger than Umbreon, so you should be thankful to have a good Pokemon like Sylveon! Guess you shouldn't have taught it that Fairy-type move. (Sorry I went on a rant there...)
Nature, EV's, IV's, etc. That's how I raise my Pokemon. Remix if you feel the same way.
