Rokoko

Lokoko (Japanese: ロココ Rokoko) appeared in "Just Waiting On A Friend" and lived in a mansion with a Ninetales. Brock fell head over heels in love with her, and was considering living with her forever. But Ash and Misty felt that something was odd. Whenever Lokoko went by a mirror, she had no reflection, and whenever they tried to alert Brock to what was happening, Lokoko and Ninetales would get in their way.
After finding an old room in the mansion, they learned that the Ninetales belonged to the owner of the mansion. He left one day hundreds of years before, and never returned. Lokoko was an illusion created by the Ninetales.
After a battle with Team Rocket, Brock learned the truth about Lokoko and Ninetales. The reason they wanted him to stay was due to his resemblance to the mansion's owner. After Ninetales's Poké Ball was broken in battle, Brock told it to go free. Once it did, the mansion and Lokoko disappeared.

(Source: Bulbapedia)