This is pretty spoiler-y. I'd recommend playing on your own but if you want to know they are.
*** Spoiler - click to reveal ***
1) The magistrate's daughter. She's in the upper cistern. You can either use Duli's key or the golden bow on the lower cisterns. She's important for a couple of endings.
2) The Hermit Philosopher. He's in the under part of the city. Through the path at the Greek Shrine
3) The Egyptian Fisherman. After finding the name of the god who has trapped you there, return to the Hermit and he will give you a key to the locked gate near by. Go thru the dungeon and he will be at the end.