So, this is somewhat more challenging than creating more drones, because now you might want to go beyond preset identities like "wheeled", "jet" or "hover" and create a spirit with totally new animations because it has got, like, tentacles or something. I wanted to create the spirit of Dog for Jake Armitage because of previous lore involving Jake and Dog.
I took the Dante model (angry dog) from Dragonfall, changed its skin a little and used Unity particle effects to create a permanent fire breath. Then I had to create new animations for PlayerBody Identity DOG that are "subject" (when adding AP to spirit passes the roll), "escape" (when it fails), "death" and "banish" (it's the same animation), then I just threw in the pre-existing animations for running and attacking from Dante. In this video you can see the spirit escaped. Finally, everything was ready to create a new Bound Dog Spirit item for Jake Armitage using prefab "David:Spirit/SpiritDog". I still haven't figured how to integrate some missing sound effects (maybe when I create a vfx bundle for my bundle group), but it's decent overall.
All sorts of realtime particle and texture effects can be running with spirit animations or baked into the spirit prefab (but that also works for any character prefab, even for outfit prefabs). For example, the water elemental model has got a 3D base that continually spews a rolling water texture. I just went for something "simpler" with just one fire particle effect that explodes when the spirit is killed or banished.