Fanboys
A fanboy will let their personal opinions guide their arguments, and make biased arguments in favor of their favorite obsession. There are different levels of fanboys, going from casual fanboys who will simply argue against the odds but still use reason and concede when defeat is inevitable, and then there are the completely insane fanboys, like for E.G itachi fapboys who will never give up, never concede, never admit that their favorites could ever lose, even if their arguments have been ripped to shreds hundreds of times by dozens of people in the same thread.
While the term "fangirl" could be used to refer to a female with a fanboy - like personality, it is more usually used to refer to female fans of certain characters who are obsessed with them sexually and want to **** them even though they're not real. Said characters popular with fangirls also tend to be Made of Fail....eg. Sasuke, Itachi, Sephiroth, Draco Malfoy, Inuyasha, Edward Cullen etc.)