Itsuki Koizumi

Itsuki Koizumi (古泉 一樹, Koizumi Itsuki?) is a fictional character from the Haruhi Suzumiya light novel series; in the anime he is voiced by Daisuke Ono in the original version, and Johnny Yong Bosch in the English dubbed edition. Itsuki is the last member to have joined the SOS Brigade, having been recruited by Haruhi as a "mysterious transfer student". Itsuki was sent to observe and keep Haruhi emotionally stable, 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.

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. Kyon himself thinks that he prefers seeing him smiling instead of looking melancholic, although the feeling makes him uncomfortable.

Out of the four Brigade members, Kyon is initially 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. However, he eventually comes to trust Itsuki, and despite his constant protests, is more often seen conversing with Itsuki than any of the other members, as Yuki is usually silent and Mikuru is abused by Haruhi most of the time. 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. Despite Kyon's less-than-enthusiastic approach to the SOS Brigade and the various external forces at work around it, Itsuki is usually the one to explain supernatural phenomena to Kyon when necessary.

Although Itsuki was named official second-in-command by Haruhi for providing the Brigade a place for their vacation/mystery investigation (Remote Island Syndrome Part I), this title appears to be little more than formality since, in every aspect, Kyon is still Haruhi's right-hand man. Itsuki is often seen fighting various Shinjin in closed spaces throughout the anime. During these instances, Itsuki displays some of his esper abilities.

Itsuki is a member in a secret association which he simply refers to as the "Agency" (機関, Kikan?, translated as the "Organization" in the English-dubbed anime). The members of this society consist of special humans known as espers who gained unique ESP-like powers and special knowledge regarding Haruhi three years ago. Since then, they have been actively protecting humanity by fighting giant beings known as Shinjin (神人?, Lit. "godlike person") within the closed spaces generated by Haruhi. Should the destruction of the Shinjin (also known as Celestials) be left unchecked, the Closed Spaces generated would eventually take reality's place. He often gives his gratitude to Kyon for keeping Haruhi emotionally stable, and in turn, reducing the workload he and his colleagues are required to perform.

Source: Wikipedia