I would say that most characters have passionate fans but 'fanatical fans'- as you define it- are relatively few.
I would say the more popular a character is then the more likely it is for there to be a subsection of that fan base that are defined as fanatics. Also if that character is particularly polarizing in some way then it tends to make the fanaticism more common, for whatever reason; I've never seen a Riley fanatic or a Tara fanatic because they are 'relatively' straight forward, uncontroversial characters
Buffy and Angel have a few who will defend them even when they do incredibly questionable things. This isn't the same as contextualizing what they did or why they did that thing, its saying 'this character did nothing wrong here' or 'well, this other character did something similar or worse so it doesn't matter'.
I would say Wesley has some fanatics back in the day, Cordelia too.
Personally I think almost every character should have somebody in their corner, no matter how unpopular they are. I have some admiration for those Connor, Dawn or Kennedy fans whose faves have plenty of dislike thrown their way but not too much in the way fanatics, its unbalanced.
Hell, a while ago I got into the habit of sticking up for a few characters- Xander or Riley for example- who sometimes get piled on without anyone championing their point of view.
I think that should be a factor in deciding what is, in fact, a fanatical fan; are they defending their favorite because they just can't deal with somebody 'attacking' their fave, or because they feel the need to because nobody else is bothering to put forward a positive view of that character, even if that positive view is a bit too skewed to be considered strictly reasonable.
