Vessels

Vessels, or Meatsuits, are humans who serve as the physical body or embodiment of spiritual, non-physical beings. At least some of these beings can be forcefully driven out through an exorcism while others can be driven out by the vessel.

Characteristics
Angels, of every variety, use possession as the means to interact with humans on Earth, and they require the vessel's permission before they can take any form of control, under a law mandated by God. Angels can bind Horsemen to their vessels, as shown by Lucifer, but if the vessel is dispossessed of the angel or is killed, the spell will break. Angels' true forms can fry humans to rotting corpses and their voices, outside vessels, can render humans deaf. Although, the angel's true vessels can withstand their respective angel's form and voices.

Angels have multiple people that can be vessels for them, but only one person is their "true" vessel. While in a lesser vessel, an angel's abilities are limited when compared with their true vessel, and, as evidenced by Lucifer and Hael, may be damaging to the human. Archangels require their true vessel, otherwise they will burn out the body they are inhabiting. Not all humans are even compatible to house angels, and those who are incompatible and house them, will quickly degenerate and explode into blood, similar to the process of Molecular Combustion.

Possessing a person has a downside as it renders them vulnerable to death by whatever they are vulnerable to by anyone that has that weapon. For example, an angel in a vessel can be killed by a human or a demon if they possess an angel blade when without a vessel, this would be impossible. When demons possess people, it also allows them to be killed when even an angel's powers are unable to kill them in their smoke form. However, the downside of this is that the person they are possessing is killed with them. If a demon's original human bones are burned, both they and their vessels are set on fire and burn to ash.

It is revealed that an angel that has become a human can themselves become possessed if they say "Yes."

Demons possess the ability to restore their destroyed vessels with a ritual.

More than one being may possess a vessel: for a short time, both Nathaniel and Hannah possessed Rebecca Greene before Rebecca expelled Hannah and Nathaniel willingly left.

According to Nathaniel, a person who is possessed can access the memories of the one possessing them and are aware of everything that being is aware of even if they don't remember it. With encouragement from Nathaniel, Rebecca is able to access a few of the memories of Hannah.

Process
It is the law that angels use vessels to interact within the physical world. But in order for this to happen, the angel must first gain consent of their would-be vessel, who must voluntarily agree to the transaction, and no angel can disobey it, and thus will only take a willing vessel and will not and can not force themselves into a human-like demons, thanks to the laws of God. Even archangels aren't exempt from this. Because of that, and the fact that most humans cannot stand the sight of an angel's true form, the angels have to find both willing and durable vessels.

After finding a potential host, the angel will then communicate with the human through visions in order to explain themselves and ask for the human's consent to gain control of their body.

It has been shown however that if the vessel has enough strength of will, they can overpower the angel for at least a short time and retake control. This is true with demons as well, but for both, the vessel must be exceptionally strong-willed. It's also been shown that an angel can choose to let their vessel take complete control and not even know that they are there but they can take control themselves when they wish without further permission.

Archangels, however, are much more powerful than regular angels and as a result, their presence in a human "hollows them out" leaving a vacant empty shell once they leave the human, with recovery unknown.

Angels can leave the host whenever they want to or if they are called away by the higher-ups, leaving the vessel with little or no memory of the time they were occupied. The process also doesn't kill the human either as angels heal their vessel's wounds completely unlike demons whose vessels have their wounds return after the demon is exorcised or leaves. Also, as angels do not need to eat, a vessel may find themselves extremely hungry after being freed from possession.

A difference between angelic and demonic vessels is that while demons are unaffected by injuries to their vessels, they don't actually heal them and when the demon leaves their vessel through choice or exorcism if the vessel has been fatally injured, the vessel will die. This tends to happen with vessels who have been possessed over a long period of time while vessels who have possessed for a much shorter period of time may survive, though traumatized. In the case of angels, when their vessels take injuries, they heal the injuries and the vessel will live if the angel leaves no matter how severe or fatal the injuries the vessel took over its possession.

A vessel of an angel, due to their consent being needed for possession, can actually expel the angel possessing them if they wish to.

Angelic Occupants
For Nathaniel: Nathaniel's vessels are applicable through the Bartholomew Lineage.
 * Jared Bartholomew - A teenage boy who was selected by both God and Nathaniel to act as God's true vessel.
 * Lisa Murray - Briefly allowed Nathaniel to possess her to save her family but he eventually left her for Jared once again.
 * Rebecca Greene- When Raziel suggested Nathaniel possess Rebecca to help her expel Hannah, Nathaniel got Hannahs permission to battle it out in her head and he won.
 * Stacy Greene - As Nathaniel can possess Rebecca, it is likely he can possess Stacy (Rebecca's sister) as well if he has her permission.
 * Unnamed Male - At one point, Nathaniel had a male vessel from an unknown lineage. Nathaniel used this vessel while protecting the garden of Eden, but it was destroyed during the war in heaven, but while in heaven, hell, or in someone's head, he appears as this vessel suggesting he preferred this vessel. He also once said that he preferred this vessel as he was the only one occupying it.
 * About 5 Different vessels - Nathaniel stated that he had burned through about 5 other vessels before possessing Jared. SPN 0387.jpg

For Michael: All descendants of Cain and Abel are capable of successfully containing Michael without discomfort. For Uriel: The requirements to host Uriel are unknown.
 * Unnamed Woman - Back in the 15th-century, Nathaniel possessed this woman to kill a rogue angel.
 * Andrew Matthews - Metaphorically the Sword of Michael, he was preordained from the birth of humanity to host Michael. Currently the vessel of Michael.
 * Henry Matthews - Since Henry is a descendant of Cain and Abel and a member of the Matthews bloodline, it's assumed that he could be a possible vessel for Michael.
 * John Matthews - He was chosen by Michael to serve as an alternate vessel so that he could save Mary Winchester (to ensure Sam and Dean were born) and converse with Dean.
 * Daniel Matthews - He was chosen by Michael as an alternate vessel.
 * Uriel's vessel - Uriel's primary vessel on the series.
 * Uriel's second vessel - Vessel briefly used by Uriel in the past.