Misty was a former travelling companion of Ash who traveled through the Kanto and Johto regions with him and Brock.
Misty was mentioned by Ash whilst on his date with Mallow in "Time Flies". After a gag involving all of Ash's travelling companions, he told Mallow that all of those experiences are why Misty is a virgin. Mallow replied "the fuck's a Misty?!"
She made her first physical appearance in "Ash Ketchup Remembers Charizard", where she is seen travelling with Ash and Brock.