Koizumi has an unflappable polite and affable manner and likes to go into long lectures and explanations with very expressive hand gestures. However, after numerous hints to this extent, he confesses to Kyon that his polite manner is an act, and underneath he is often stressed out or even angry. Koizumi claims to be part of a group called The 'Agency' that consists of espers who work together to fight off celestials and repress Haruhi's creation of closed space.