Itsuki Koizumi

Height: 178 cm (5'10")
Occupation: Student (grade 11), Vice Commander, Esper

Itsuki is the last member to have joined the SOS Brigade, having been recruited by Haruhi Suzumiya 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's fondness for speculation often causes him to produce explanations for situations involving the SOS Brigade, its members and related events. Itsuki only tends to share these explanations and 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.

Itsuki is an Esper, and was sent to observe and keep Haruhi emotionally stable by a mysterious group known as "The Agency", in order to protect the rest of humanity. Although he is an esper, which is exactly one of the sorts of people Haruhi desires to meet, Itsuki never reveals himself as such, at least, not to her. Despite the theory he advances in which Haruhi is the equivalent of God, Itsuki says that there are many differing lines of speculation about Haruhi within "The Agency." Itsuki himself does not believe in a singular God, nor does he regard Haruhi as such, although he reveals that he has colleagues that do. Apparently, most of the questions that he raises about Haruhi's true nature are philosophical in nature and do not necessarily represent his personal beliefs. Itsuki also reveals that the majority of "The Agency" advocates observing Haruhi instead of doing anything that might provoke her.

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. He is also distrustful of Itsuki's intentions, comparing him to Yuki Nagato when it comes to deciphering his expressions.