One of the classic and most common character types in gaming is the generalist, a jack-of-all-trades with capabilities in all fields and no particular weaknesses. But specialization has advantages, so it takes a deft touch to ensure that the generalist has reasons for being used. Sometimes it simply doesn't work. They aren't completely useless at anything they try, but they're not good at it either. They have no particular weakness or vulnerability, but that's offset by the fact that they're vulnerable to everything. That character is a Master Of None.