The princesses rotate: Cinderella, Mulan, Pocohontas, Sleeping Beauty, Belle, Snow White, Jasmine. Of course, Ariel is always there. When we were there we saw Fairy Godmother and a Prince. I belive Wendy sometimes makes the rounds there. Also, I saw a post where someone had seen Princess Minnie at Ariels and she was very cute. I don't know if you can call the restaurant to ask who will be on the schedule for the time you will be there.
I just suggested this in another post -- if there are other princesses you want to see (that you don't get to see at Ariel's) you can head to Princess Fantasy Faire (long wait). Jasmine is at Aladdins Oasis Storytime. I think Pocohontas is sometimes in Frontierland. Now that they have canceled the princess storytime and the princess meet and greets and the castle, it is getting a bit harder to find them outside of the Fantasy Faire and character breakfasts.