I agree. There's so much to explore about him. For instance, what does he do at home? Does he have any other friends? What's the Organization like? What is the real Koizumi, since he's admitted that his persona is simply for show? After nine novels, we still don't know too much about him. Everyone else in the SOS Brigade has had significant plot arcs largely devoted to them. (Haruhi in book 1, Yuki in book 4, Mikuru in book 7) Therefore, it's hypothesized that the current arc that spans novels 9 and 10 will feature Koizumi prominently.

But alas, people are too preoccupied with who goes with who and what goes in which hole. >_>