Not to pull your halo down.

In the Bible, only three angels are mentioned: Michael, Gabriel, and Raphael. Lucifer, of course, being a fourth fallen angel, but more commonly known as the Devil. However, it is known that there is a great number of angels besides them.

The character was inspired in the Judeo-Christian archangel Cassiel:

« Cassiel (Hebrew קפציאל Qafsiel Kaziel) is the Latin name of an archangel in post-biblical Judeo-Christian religion, particularly that of the Kabbalah. Unlike many other angels, Cassiel is known for simply watching the events of the cosmos unfold with little interference. He is the angel of solitude and tears, and is said to preside over the deaths of kings. He is often included in lists as being one of the seven archangels and often associated with the Seventh Heaven. In Jewish mysticism and Mesopotamian mythology, Cassiel is associated with the planet Saturn and the direction north. He is the controller of the moon. In the magical text Berit Menuchah, Cassiel is associated with Kefitzat Haderech, the ability to travel quickly through space. Magic spells using his name are cast to create destruction, to scatter crowds, to cause a person to wander aimlessly, or to fall from a position of power. Ancient Hebrew amulets bearing his name are used to drive away one's enemies. The words on the charm are written with the blood of a bird and then tied to the foot of a dove. The dove is then set to flight, taking one's enemy with it. Should the bird refuse to fly, it is a sign that one's enemies won't depart either. » [X]

If your character is not biblical, it is only normal if they don't recognize the name, as Castiel would not normally be a known angel. However, if your character is biblical or religious, it is up to you whether or not they recognize Castiel by name.

◦◦◦◦◦◦◦

Links:

What does the Bible teach about angels?
Angels' Wikipedia page
Cassiel Wikipedia page
Curiosities about the Angel Castiel