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.
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, 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 are observing Haruhi instead of doing anything that might provoke her.
Itsuki has also revealed that there are other factions that have not yet been revealed by Haruhi, and that "The Agency" is often in conflict with the other factions backing their respective agents within the SOS Brigade (not to mention various groups which have either not put in an appearance or appear to be enemies). For example, he suspects that Mikuru was chosen by the time-travelers for her beauty and for her ability to seduce Kyon, in order to control the group dynamic within the SOS Brigade, although Itsuki later suggests that he had been lying about this suspicion.
Relationship with other SOS Brigade members
Haruhi recruited Itsuki to the SOS Brigade as the stock "mysterious transfer student" manga character. Itsuki's relationship with Haruhi seems to comprise of him agreeing with her every demand and action no matter how ridiculous it is. This has been stated by Kyon himself, who called him Haruhi's 'yes-man'. During the events of the light novel The Vanishment of Haruhi Suzumiya, in the alternate world created by Yuki where everyone has normal lives and no memory of the SOS Brigade (with the exception of Kyon), Itsuki is a normal transfer student in the elite Kōyōen School (in the original world it is a prestigious girl's school), and claims that he fancies Haruhi, who is his classmate. He is also concerned with Haruhi's well being in the normal reality. It could be due to the fact that Closed Spaces appear when she is upset, but it could also be his genuine concern of her, as he had observed her with "The Agency" during the three years prior to Haruhi meeting Kyon. At one time in the light novel, Itsuki actually snapped at Kyon when he commented that Haruhi only causes trouble for everyone, even when she is asleep.
Itsuki seems to be the only one in the SOS Brigade that observes Yuki almost as much as Kyon. Whenever an extraordinary event occurs, he formulates his own theories and tries to enter into discussion with Yuki. Itsuki seems to get along with Mikuru Asahina; however the organizations in which he and Mikuru belong to have conflicting ideas regarding Haruhi. Mikuru even personally went to Kyon's class with Tsuruya to ask Kyon to disbelieve Itsuki's theories regarding Haruhi. Itsuki also seems to distrust Mikuru's future self as he tells Kyon to be wary of her.
Out of all SOS Brigade's members, Itsuki gets along with Kyon the most and even tells him that he is a bit mindful of how he is seen through his eyes and that he possesses likeable traits. Although Kyon gets annoyed with him most of the time, he often discusses his concerns with him. At times, he even shows Kyon his true self behind the smiling facade, and only Kyon notices when Itsuki is particularly upset or concerned with things. Itsuki constantly tries to get Kyon to realize Kyon's importance to Haruhi, and has expressed amazement and surprise that Kyon does not realize this fact yet. He has told Kyon that he wishes for the time when he could be his friend normally, where his duties as an Esper wouldn't matter. However, when Kyon shrewdly asks him whether he will truly be satisfied with that, he avoids the subject.
it:Itsuki Koizumi zh:涼宮春日系列角色列表#古泉一樹