J. B. Warner already said this quite nicely, but I wanted to just point out how correct he is anyway. They replaced Misty because they needed a new female character to promote the then new games of Ruby and Sapphire, as well as Pokemon Contests. May would be a much better choice not only for promoting the R/S/E games, but since Misty wouldn't work for a Contest battle like style. The same thing happened when Dawn came into the picture. I personally like May over Dawn, but I could like her better once she gets more development. Besides that, I fully know that they made that change to promote the games. Pokemon is a commercial for the video games. I don't see anything wrong with that, of course, but it's a pretty well-known fact.
By the way, the girl you couldn't remember the name of is Dawn. She only has been in the series for almost two seasons so I don't think much of anyone outside of the Pokemon fan base knows much of anything about her. I also respectively disagree with you about Misty being more interesting than both May and Dawn. Even in her season one and two days, I don't think Misty did anything as interesting as May or even Dawn, despite how she's still relatively new to the series, have done so far. At the very least, most every Pokemon fan agrees that she was completely boring and useless in Johto. I don't want to start anything, but I just wanted to express my opinion respectively as well.