Itsuki Koizumi

Itsuki is the last member to have joined the SOS Brigade, having been recruited by Haruhi as a "mysterious transfer student."

Itsuki is constantly smiling and always speaks in a formal and polite manner. He is also prone to giving long and detailed explanations, usually illustrated by exaggerated hand gestures. Itsuki has confirmed to Kyon once that his outer personality is merely an artificial image he fakes in accordance to Haruhi's impression of him, and hints that his true character is something quite different, being "less pleasing" in the eyes of others. Maintaining his casual, constantly smiling self, however, is vital for keeping Haruhi's mentality balanced.

Out of the three Brigade members, Kyon is the least fond of Itsuki. He often finds himself annoyed with Itsuki's plastic smile, his willingness to acquiesce to Haruhi's outrageous schemes, playful teasing and his various forays into his psychological and philosophical theories (e.g., omphalos hypothesis, Anthropic principle, etc.). He is also distrustful of Itsuki's intentions, comparing him to Yuki Nagato when it comes to deciphering his expressions. Itsuki's fondness for speculation often causes him to produce explanations for situations involving the SOS Brigade, its members and related events. Ironically, Itsuki can only share his theories with Kyon and also enthusiastically plays different kinds of board games with him to pass the time, despite losing all the time, although Kyon suspects that he may be doing so on purpose.