The unknown factor that makes a person attractive, interesting, or/and desirable.
Does not imply outward beauty, though it can.
It is an unknown aura that when asked to explain why one is attracted to another, there is no answer.
Can also refer to someone who has a certain charm, mystique that makes people attracted to them, or consider them to be at the top of the desirability chain.
Also a more extreme game show, where an unknown challenge, or rule, is thrown into the mix, to add to the excitement and challenge level.