There is no requirement that a Confirmation name be the name of a saint.
It can be any name that expresses the Christian faith. Indeed, many names that are now saints names began as expressions of the faith first. A good example of this is the 3 sisters who were names “Faith, Hope, and Charity” and who later became saints. Many such names began as expressions of the Faith: Athanasius means resurrection (literally immortal or undead).
I can think of many names used at Confirmation over the years that are not strictly-speaking given names of saints: Immaculata, Conception, Angel, Guadalupe, Chastity, etc. etc.
Picking a Confirmation name and picking a patron saint are not necessarily the same thing, even though we often combine those two ideas.
Choosing the name “Cassiel” as a confirmation name to express “speed of God” (the meaning of the name) is not a problem.
If someone wanted a confirmation name “Lumen Christi” to express the Light of Christ, that would not be a problem, even though there’s no canonized saint with that name (not that I’m aware, anyway). Just an example.
However, it would be problematic to choose the name Cassiel as a patron saint because there is no such saint; and indeed, the name is used by the occult—making this a very non-Christian choice.