Eve

Little is known to date about her, other than that the monsters call her Mother (as she is the Mother of All), and she calls herself Eve, and has been trapped in Purgatory, where the souls of supernatural beings go, and that she may have been the creator of the Alphas, or first monsters. Despite taking a human vessel her true visage or a portion of it can be revealed on a camera recording. The full extent of her powers and abilities, including her position in the heirarchy of supernatural beings, are thus far unkown. Although it is doubtful she is anywhere near as powerful as God, she is the only other supernatural being besides him known to be capable of creating new, original life.

Family Matters
She is first referenced by the Alpha Vampire, who is being interrogated by Samuel Campbell. When Dean and Sam have a few moments alone with him, Sam asks, "But if you're the first, who made you?" The Alpha Vampire answers, "Well, we all have our mothers. Even me."

Like A Virgin
Dragons are abducting young women (virgins) and keeping them captive in a sewer. Dean obtains the Sword of Brunswick and he and Sam track down the dragon's lair, finding a pile of gold watches, and the women. They also find an ancient manuscript which speaks of freeing Mother from Purgatory. Sam and Dean kill one of the Dragons but one escapes. He meets up with another dragon who has also captured a half dozen women. They take one and use an incantation to open a door to Purgatory. They throw the woman in, and she rises again giving form to Eve.

...And Then There Were None
Eve creates a being - called colloquially the Khan Worm - that can enter a person's body through the ear and control their actions. She plants it inside a trucker, who goes on to kill his family. It then passes to one of his co-workers who kills 12 people. While investigating the murders, it infects Dean who kills his cousin Gwen, and then infects Samuel who was killed by Sam, and finally Bobby who kills Rufus while possessed. While possessing Bobby, it tells Dean and Sam that it was created by Eve, who intends for supernatural beings to take over the world.

Trivia

 * A virgin is needed for sacrifice so as to release her from Purgatory.
 * She is played by Julia Maxwell.
 * She is the main antagonist of the second half of the sixth season.
 * It's possible that she is Echidna who, in Greek mythology, was known as "The Mother of All Monsters", however she calls herself Eve, who in the Old Testament of the Bible was the first woman to have been created, as well as the first woman to have disobeyed God. This does not count Lilith, supposedly a woman created alongside Adam who later left him and went on to have children with demons, because she is not universally accepted in Abrahamic religions.
 * There may be a distinct connection between Mother and the Winchester brothers. She calls herself Eve while Sam and Dean are blood descendants of Cain and Abel.
 * Confirmed fact: Eve is more then 10.000 years old.

Appearances

 * Season 6
 * Like A Virgin
 * ...And Then There Were None