Michael

Michael is the most powerful Archangel and, possibly other than God Himself, he may be the second most powerful being in the universe. He, along with Lucifer, Gabriel, and Raphael, is one of the four angels who have seen God personally. He is God's Lieutenant and commander of the Heavenly Host; he is the one who personally banished Lucifer from Heaven during the last war and possibly the only one who can defeat him again.

Michael's vessel is Dean Winchester, but Dean has refused to allow Michael to posses him. Without Michael or God, Lucifer will rule the world as he sees fit.

Season 5
Michael makes his first appearance in "The Song Remains the Same". After Anna Milton kills Sam Winchester by stabbing him in the stomach, Michael appears before John Winchester and tells him that he can save his wife Mary if he allows him to take possession of his body; John says "Yes," allowing Michael to inhabit his body. When Anna is about to kill Mary, Michael calls out to Anna. Anna turns around and immediately realizes that Michael is possessing John. Michael lays his hand on Anna and kills her by setting her ablaze. Uriel then calls out to Michael and apologizes, saying that "He didn't know," most likely referring to that he didn't know Dean was Michael's true vessel. Michael says goodbye to Uriel and sends him away with a snap of his fingers.

Michael then turns to Dean and says that this conversation is long overdue. Dean demands that Michael heal Sam, but Michael tells him that they need to talk first. Dean asks Michael how he could possess his father since he was his "one and only" vessel. Michael corrects Dean stating that he is his true vessel and that he can inhabit the body of all those that share the Winchester bloodline due to the fact that he and John were descendants of Cain and Abel. He then explains to Dean that even though Lucifer defied their Father and betrayed him, he doesn't want to kill him as much as Dean wouldn't want to kill Sam. Michael then says regardless of the fact that he raised Lucifer and still loved him, he is going to listen to his Father and kill Lucifer. He then explains to Dean that he will eventually say "Yes" to him because free will is an illusion and that his choices, regardless of what they may be, will lead him a step closer to his destiny that was planned out by God.

Michael tells a saddened Dean to cheer up because, unlike his brothers, he won't leave him a drooling mess after he's done "wearing him". He then tells Dean that he will erase John and Mary's memories of the whole incident and that they won't remember anything about angels, him, or Sam. Michael walks over to Sam's body, heals him, and then sends him back to his own time. Michael then walks to Dean, telling him that he'll see him soon before sending him back to his own time.

Known powers
Currently this is unknown, but it could be assumed that Michael is the strongest angel under God's command. His powers almost definitely exceed those of Lucifer, ergo he will be virtually omnipotent (all-powerful) and omniscient (all-knowing). He is often seen as the second most powerful being in creation, second only to God. Michael's true form would almost definitely kill any human. As an archangel, Michael possesses holy white light that would probably kill anything that got in his way.

Relationships

 * God (Father)
 * Lucifer, Raphael, Gabriel (Archangel Brethren)
 * Other Angels (Brothers and Sisters)
 * Dean Winchester (True Vessel)
 * John Winchester (Temporary Vessel)
 * Sam Winchester (Younger Brother's True Vessel; Possible Temporary Vessel) – Due to Sam being a son of John Winchester, that makes him a descendant of Cain and Abel, just like his older brother and father, meaning that it may be possible for Michael to use Sam as a temporary vessel even though he's Lucifer's true vessel)
 * Adam Milligan (Possible Vessel) - When Adam was resurrected by the angels, it was confirmed by Castiel in "Point of No Return" that because Adam has the blood of John Winchester flowing through his veins thus making him a descendant of Cain and Abel and also he is a brother of Sam it wasn't perfect, but it was possible for him to be a vessel for Michael

Season 2

 * "Houses of the Holy" (first mentioned)

Season 5

 * "The Song Remains the Same" (first appearance, possessing John Winchester)
 * "Point of No Return" (doesn't actually appear, but his presence is known)

Trivia

 * His name means "who is like God."
 * Michael is considered the "viceroy of Heaven," meaning the ruler of Heaven in God's stead.
 * He is the only angel in the Bible named as an Archangel; the rest were named in different interpretations of the Bible and other biblical scrolls, or given the title by the Catholic Church. Hence why he is considered THE Archangel.
 * Many exorcisms invoke the name of the Archangel Michael, to defeat and banish the demon.
 * In the final judgment, Michael is shown as the weigher of souls.
 * Michael is the only angel who is also a saint.
 * Michael holds the keys to Heaven and Hell.
 * He is the Archistratege of God ('Archistratege' being a military rank of the highest degree, superior to a field marshal or grand admiral).
 * Michael is known as the 'Prince of the Seraphim' (the Seraphim being the highest order of angels).
 * He is often shown as the angel who spoke to Moses as a burning bush, delivered the Ten Commandments to Moses, and fought with Lucifer over Moses' body and won.
 * In the Apocalypse, Michael leads the army of Heaven and defeats Lucifer and the Antichrist.