Thanks! :)
I used modular sprite assets, ie. combine hair sprite, clothes, body, ears, etc... to make new characters! And I drew the portraits.
For the AI I have them "talk" in the background to cache the answers whenever possible. Eg. If you have a scene where "speaking order" goes : Player -> Noire -> Lunara -> Noire -> Lunara -> Player, as soon as the player speak and the game receive Noire's answer, I send it to Lunara in the background, then her answer to Noire, ... so by the time the player click to go to the second line I already have stored up to the 3rd or 4th, basically.