bakaneko said:
who are the extras?

Naked_Flame said:
*Has read about them on Wikipedia about two years ago and doesn't remember full details, like names.*
spoilers!

They are all counterparts of the characters they are standing next to. They exist as familiars to another 'god' (like how Mikuru, Yuki and Itsuki are 'familiars' of Haruhi). The other 'god' is an old friend of Kyon's.
The guy standing next to Mikuru is from the future - he is a member of a group of activists who are campaigning against time travel.
The girl standing next to Itsuki is another esper - from a rival clan of espers who believe that it was the other 'god' who caused the surge in data from however many years ago it was.
The loli with the incredible hair standing next to Yuki is from another Integrated Data Thought Entity (named something different) - which shocked Yuki a little because it was believed they couldn't talk (neither IDTE could communicate with the other). She tried to kill Yuki once...

I think they're from the most recent light novels - they haven't appeared in either anime or manga yet.