From Inner.org: "The sefira of keter develops into two partzufim (personaes). Its external partzuf referred to as Arich Anpin ("the Elongated Face"), corresponds to the superconscious power of ratzon (will) in the soul. Its inner partzuf, referred to as Atik Yomin ("the Ancient of Days") corresponds to the corresponding power of ta'anug (pleasure) in the soul).
The seven lower sefirot of Atik Yomin are enclothed within the partzuf of Arich Anpin (thus introducing pleasure as a primary motivational force driving the will). The three upper sefirot of Atik Yomin, which are not enclothed within Arich Anpin represent the unadulterated superconsciousness pleasure of emunah (faith)--deriving from one's essential rootedness within Divinity.
The meaning "crown" corresponds, in particular to "the head of nothingness," the source of Divine pleasure in the soul. The "crown" symbolizes the spiritual power to receive and integrated into one's consciousness the pleasure of Divine revelation, as our sages express with regard to the world to come: "the righteous sit with their crowns on their heads and receive pleasure from the radiance of the Divine Prescence" (Tractate Kalah Rabati 2:3)".
Wikipedia: "Keter is so sublime, it is called in the Zohar "the most hidden of all hidden things", and it is completely incomprehensible to man. It is also described as absolute compassion, and Rabbi Moshe Corovero describes it as the source of the 13 Supernal Attributes of Mercy.
Keter, although being the highest Sehphira of its world, receives from the Sephira of Malkuth of the domain above it (see Sephirot). The uppermost Keter sits below no other Sephira, although it is below Or Ein Soph which is the source of all Sephirot.
According to the Bahir:
The first Sephirah is called the Crown, since a crown is worn above the head. The Crown therefore refers to things that are above the mind's abilities of comprehension. Additionally, all of the other Sephirot are likened to the body which starts with the head and wends its way down into action. But the crown of a king lies above the head and connects the concept of "monarchy", which is abstract and intangible, with the tangible and concrete head of the king.
13 Supernal Attributes of Mercy
Rabbi Moshe Cordovero, in The Palm of Devorah, discusses ethical behaviour that man should follow, related to the qualities of the Sephirot, in order that man might emulate his creator. Through discussion of a line in Micah, 13 attributes are associated with the Sephira Keter:
Whos is God lik you, who pardons iniquity and forgives the transgressions of the remnant of his heritage? He does not maintain His anger forever, for He delights in kindness. He will again show us compassion, He will vanquish our iniquities, and You will cast all their sins into the depths of the sea. Show faithfulness to Ya'akov, kindness to Abraham, which You have sworn to our fathers from days of old. (Micah 7:18-20) Non-Jewish Practices
In The Mystical Kabbalah, Dion Fortune describes Keter as pure consciousness, beyond all categories, timeless, a point that crystallizes out of the Ein Soph, and commences the process of emanation that ends in Malkuth.
The name of God given to it is Eheieh, the archangel that presides over it is Metatron, the order of angels that reside in it are the Holy Living Creatures (the Chaioth ha Qadesh), and its mundane chakra is said to be the First Swirlings of the cosmos (Primum Mobile, Rashith ha Gilgalin).
As pure formless consciousness, it is often compared with the Sahasrara chakra, in Indian Shakta Tantra.
In Aleister Crowley's Liber 777, Keter is associated with the Four Aces of the Tarot deck, White, Brilliance, Poseidon, Brahma, Wodan, Zeus, The Trinity, Almond in flower, Diamond, Elixir Vitae, Dao, and Death (not a complete list of te 777 associations).
It is said to have a negative aspect, the Qliphoth Thaumiel.
Keter is also indentified with the planet Neptune, the Atma in Theosophy and Raja Yoga, and the Khabs am Pekht in Egyptian mysticism."