Koizumi is the fifth person to join the SOS Brigade, and was recruited by Haruhi Suzumiya because he fit her idea of a "mysterious transfer student", after he was transferred into North High. He later reveals himself to Kyon as an esper, working for a group he only acknowledges as the 'Agency' (translated as The 'Organization' in the anime). According to Kyoko Tachibana, he in fact founded it. Though he was sent to North High by the 'Agency' to observe Haruhi, he believes that it was Haruhi's will that really brought him there. Due to his "distinguished service and dedication to the brigade," Haruhi promotes Koizumi to Deputy Chief. 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. He can often be seen playing a board Game with Kyon while he's talking or explaining something to him.
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