Unknown is the element of all monsters and characters whose element varies, depending upon the player's choices, or some chance-related event. It's not technically an element, but rather a Pseudo Element (more technically, a Made-up Element).
Technically, no character is of the element Unknown, however, some characters are customisable, and their element changes depending upon the player's actions. Likewise, no monster is of the element Unknown, however, some monsters mimic your character's elements.
People and Monsters of the Unknown element