See also the relevant category page: Category:Vessels
A vessel, otherwise known as a host or human form (and in the case of the Eyes of Mikhail and Raphael, a bearer or a wielder) is a human body that a spirit or spiritual being possesses.
All inhabitants of the mortal world need both a body and a soul in order to live. Spirits and gods possess humans and inhabit human bodies in order to be able to interact with the mortal world.
The Seven Ghosts, Mikhail and Raphael are highly selective when choosing their vessels. Mikhail has been shown to have no issues with killing a vessel if he deems the vessel not pure enough (as shown when the Eye of Mikhail is forced into a Warsfeil, resulting in the Warsfeil's death). The Seven Ghosts only choose vessels who are blood descendants of the Seven Houses of God, and only the Empress or Empress-to-be of Barsburg can be a vessel for Raphael.
Gender and personality also seem to be important factors in choosing a vessel, for the Ghosts, Verloren and the two Archangels. There is evidence to suggest that the Ghosts and Verloren are more comfortable using male vessels.  On the other hand, Raphael only uses female vessels. Mikhail's preferences about the gender of his vessels is less clear-cut, though it's worth noting that his two most recent vessels are both male. The personalities of the current generation of Ghosts seem to be rather similar to the personalities of the previous generation (e.g. the Raggs War Fest was knowledgeable, like Castor, and Guido was hotheaded but kind, like Frau), and Ayanami's personality is very similar to Verloren's (at least before Verloren's fall from grace).
Mikhail and Raphael are devoted and loyal to pure vessels or vessels with 'beautiful souls', but will kill or leave vessels that are impure or weak. Kors apparently do not care for the humans they possess, and leave their vessels in a deteriorated state.
- Only one female Ghost vessel has appeared in the series so far, and Verloren's current vessel is male.