stillsostrange

I don't know if I'd go with Keres, since that makes me think of crow-like battle spirits, hovering over a battlefield--very externally oriented, rather than actually entering someone. Although, Wikipedia lists, "tenebrae" (Darknesses) as an alternate term for them. That could be cool.
Also, if you want to go sideways from Greco-Latin, you could do something with the nefesh (http://www.aish.com/literacy/concepts/the_soul.asp), which is the Hebrew term for the more animalistic part of the soul that resides in the body.