Kylira ( the other one character who travel with turtle), Trudy, Urka and the Warlock ( Dark Wizzard from second town) are the current available companions.
Dullhan is a planned companion if there are to be more you'll just have to wait and see.
Vampires 1 of the art pieces is completing the Vampire Hunters quest. Which is buying a Cod piece at the Outpost for 100g and bringing it back to her. Another piece of Art for the Vamps is either losing or defeating them in their third fight. Losing or defeating them will determine how the scene plays out.
Some of the characters can only be seen by doing certain things or going certain places. Example the Unicorn is only visible if you go to a space that has Centaurs and you're an Anal virgin. There are also other areas to visit like past the Outpost or "North" of the Alter spawn.