Cynthia is the Champion over in the Sinnoh region, but she likes to take vacations over in Unova. You'll find her waiting in Undella Town.
Cynthia can be found over in Caitlin's Villa, in the western part of Undella Town, after beating the game. It doesn't matter what season it is the first time you visit her, so don't worry, but make sure you've got a strong team before entering the house, because she is very challenging!
||PKMN Trainer Cynthia||$15600|
Her team is varied and has very little in the way of shared weaknesses, making her a challenging Trainer indeed!
After beating her the first time, you can challenge her again each day, but only during the spring months; even though she's around the villa in the summer, she won't battle you.