Itsuki Koizumi is the fifth person to join the SOS Brigade, recruited by Haruhi Suzumiya because he fit with 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). 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. (Edited from the Haruhi Suzumiya wiki)