Castelia City
She is located in the first building on the left side of Castelia Street.
>Castelia Street has many people coming and going, especially during the evenings when people are going home from work. This street has many tall skyscrapers and large buildings; however, it features the Game Freak HQ and a building where one Pokémon per day can receive a massage to increase its friendship.
Source
Team AB.
&
Team Swagg.
