Now this, I guess, also could be quantified
as the type of person that would pretend to be HOTFEM18 and is in actuality a fat bald guy
but for me it's a little bit different.
For me IRC acting is a great sport.
The other night, as an example, I was on #chatzone on EFnet pretending to be Nicole19 (a right wing christian who was beaten by her father while he was an alcoholic a few years ago) and Wilber (a jerk off macho guy who is homophobic to the point where you begin to question his sexuality).
It starts out very innocent, but you begin to get deeper insight into the characters as the chat progresses, revealing wierd things about themselves. I once convinced a room that I was an albino midget. (it also helps if your character joins channels that help to define them, such as #christian for Nicole19 and #teensex for Wilber, so that anyone doing a /whois may notice) Nicole19 was very psychologically unhealthy, some of the other people on the channel where trying to help her by consoling her. You might think that this is mean or underhanded but it is great fun and not vindictive. It is also fun to coordinate your friends onto a channel like #teenchat. The tactics you use must be very subtle, at least enough to be believable. One character may start to attack someone on the channel while the other will defend them. It got the point where a full blown argument on religion was taking place based on one of my character's narrow view of religion. People where /msg'ing both of the characters with radically different views and conflict abounded, it was fantastic.
I'll post some of the transcripts from chats that i'll do in the future.