Wow, the first Summon Night is really? Is a bit difficult for me sometimes, and I think that I have to go many times for level the characters, maybe I am the problem (I am talking about the PS1 Version)
In the PSP Version of Summon Night 4, you can play with Aty if you have a save file from Summon Night 3 in the console with she as the protagonist. She will in the Puppet System.