Maria is Harry's lover while he's getting ready for the ball. It turns out she is lots of people's lovers; in fact, that's how she makes her money:
It was not my fortune to be Maria's only lover, nor even her favorite one. I was one of many. Often she had no time for me, often only an hour at midday, seldom a night. She took no money from me. Hermine saw to that. (385)
Maria ends up being like an object, a gift from Hermine to Harry. She sort of stands in for Hermine while Harry is still learning how to love properly, and gets him ready for the real deal. The fact that all of these lovers are in league together lets us think that Harry's lesson is one big conspiracy—Pablo, Hermine, and Maria are all working together to get him ready for a new, sexy, kind outlook on life.