Cassiel is a holy archangel within the Judeo-Christian faith, and is named within noncanonical Biblical scripture, as well as in the Jewish Kabbalah. The name is Latin in origin, and is also spelled as Qafsiel occasionally. What is strange about Cassiel within scripture, is that he is not affiliated with any particular duties or angel like attributes as far as mainstream Judaism and Christianity are concerned. Instead it is mentioned that as an archangel he is simply a watcher, bearing witness to all of the events that unfold within our universe, or in God's creation. It is said that Cassiel if forbidden to interfere with any of the events that he is witnessing unfold. What is even stranger, is that scripture goes on to state that Cassiel is burdened with solitude and tears, and that his presence is bountiful upon the death or passing of any great ruler or king. So I guess in this sense he is sort of a melancholy archangel, differing greatly from his archangel brothers and sisters.
With regard to Christian or Jewish inspired occult beliefs, Cassiel tends to take on a different role as compared to the mainstream role mentioned above. For example New Age groups claim that Cassiel is the angel of fire, and rules over lesser angels of fire. It also goes on to claim that Cassiel very much enjoys the company of us humans, almost preferring them over the company of his angelic brothers and sisters. It is also believed that he is the guardian of Saturday. These angelic occultists also believe that you can pray to Cassiel when you lack balance in your life, or when you feel totally overwhelmed by stress. Apparently you will know that he's answered your prayers by a wave of peace that will wash over you upon saying amen. Obviously all Christian or Judaic beliefs would considered these attributes just outlined to be totally false, and would strongly condemn praying to anyone other than God the Father, God the Son, or the Holy Spirit.
Cassiel has also appeared as a figure in many works of fiction. For example Castiel in the TV series Supernatural is loosely based on the archangel Cassiel, and is played by Misha Collins. Obviously in this case the spelling of his name is slightly different, however it has been confirmed by the show's producers that the character is indeed modeled after Cassiel. Like scripture that describes Cassiel, Supernatural's version is somewhat of a melancholy figure. Outside of Supernatural, Cassiel is also a fictional character in the film Wings of Desire by Wim Wenders, as well as the film's remake entitled City of Angels. Cassiel in this case is played by Otto Sander in the original, and then by Andre Braugher in the remake. In the episode called Tough Love in Buffy the Vampire Slayer, Cassiel is mentioned in a holding spell that Willow places on Glory. Outside of film or TV, Cassiel appears in many PC games, and even in a few anime comic books.
In the world of art, you are not going to find many traditional Catholic or Eastern Orthodox representations of Cassiel, as you would with any of the other more well known holy seven archangels. I researched this pretty thoroughly on both the internet, and in the public library, and very little came up. I was not able to find any medieval representations of Cassiel at all, with the few that I found consisting of a few New Age contemporary paintings and sculptures. This leads me to believe that prior to recent times, Cassiel was hardly ever mentioned albeit in the scarce few noncanonical scriptural passages, and played no role in Christian or Judaic rites or iconography. This is not surprising, as the lesser known archangels outside of the famous seven, are very difficult to gather actual scriptural information on. My guess is that Cassiel is only recently celebrated via modern mediums such as film and anime, and prior was confined to the shadows.