Kyon: a laid back dry-witted guy who gets wrapped up into the insanity; likes curvy moes. Also the only one with enough balls to stand up to Haruhi. So cool a genderswap had to be made so people wouldn't feel so gay for Kyon. Also famed for one doujin where he does what some folks who are fed up with Haruhi's antics do.
Kyonko: Said genderswap that has a frame similar to Yuki's, and is on the tsundere side. Very popular with OT.
Mikuru: First modern day curvy moe. She's a time traveler who's sent to observe Haruhi (and gets harassed royally in the process). Personality wise she's rather moe blobish, but with an apparently strong resolve if she tolerates such things. Her older self is more confident and sexier. She inspired StriderTuna to make the term curvy moe.
Yuki: Stoic bookworm cutie that's actually an alien, is a victim of a stupid meme. Quite popular in places, especially those with lolicons.
Itsuki: Rather fabulous esper. He seems a bit too interested in Kyon. He always smiling and it's impossible to tell what he truly thinks.