Kyoko Tachibana

Kyoko Tachibana (橘京子 Tachibana Kyōko), like Itsuki Koizumi, is an esper. But unlike him, she believes Sasaki rather than Haruhi Suzumiya to be the cause of their abilities.

Biography
Tachibana is speculated to be near the same age as the SOS Brigade and Kyon describes her as a woman of his age. (In The Dissociation of Haruhi Suzumiya, she is most likely in her second year of high school.)

At around the same time the 'Agency' formed, a "rival" group formed, including people such as Kyoko Tachibana. Her group firmly believed that their powers had, in fact, come from Sasaki, and went searching for her. Conflict broke out between the two groups, and despite numerous meetings neither could convince the other side. The rivals believe that the powers were accidentally transferred to Haruhi Suzumiya.

Tachibana first appears alongside Fujiwara in The Intrigues of Haruhi Suzumiya, in which she helps kidnap Mikuru Asahina; while this plan fails, she convinces Fujiwara, a time traveler, to stay as an ally.

She was unable to transfer to North High out of fear of 'Agency' member Sonou Mori.

Tachibana believed her group needed the support of a time traveler and alien, just like the SOS Brigade, and was eventually able to assemble the group.

In The Dissociation Sasaki took Tachibana and Kuyou Suou to meet Kyon on Saturday morning, just before he was scheduled to meet the rest of the SOS Brigade. Tachibana seemed delighted to have met with Itsuki Koizumi.

Plot
According to Itsuki Koizumi, Tachibana is the leader of her organization. He discussed this with Kyon in a phone call.

Beta Plot
In the beta plot, Sasaki called Kyon for a meeting on Saturday evening with the rest of her compatriots. Despite misgivings, Kyon consented to the meeting.

Tachibana tries to convince Kyon that he should transplant Haruhi's powers into Sasaki as she believed Sasaki had a more stable personality, but confessed that the other members of the group, even Sasaki, showed little interest in this. She tried to convince Kyon by showing him her closed space which she said was "calmer than Haruhi's," but when he does not take well to the idea, she becomes very annoyed.

She is shown to be the most enthusiastic about making Sasaki the one with the godlike powers and is most likely to be the most progressive in attempting so.

She also claims that the incident taking place in The Intrigues of Haruhi Suzumiya was a mistake on her part, adding that she was influenced by Fujiwara.

After Yuki Nagato fell ill, Kyon demanded another meeting, which was held Tuesday after school. Tachibana tried to smooth over friction in her own group, but it didn't really work. Fujiwara insulted her repeatedly. Kyon refused to fall for Fujiwara's blackmail, but Tachibana wasn't ready to give up. She suggested that if Haruhi lost her powers, the rest of the Brigade would disband, but Kyon could hang out with Sasaki and still enjoy a supernatural-affected life. She told Kyon that Koizumi founded the 'Agency' and this action would relieve his burden.

Even when Kyon hadn't reconsidered, Tachibana left the meeting saying she would give him time to think it over. Once Sasaki and Kyon eventually agreed to meet one more time on Friday, Tachibana hailed a taxi to bring her, Fujiwara, Sasaki and Kyon back to North High. They stepped out of the taxi in Sasaki's closed space, with Sasaki nowhere to be seen.

Combined Timeline
After the timelines merged, Tachibana was shocked to see Koizumi arrive to aid Kyon, as he shouldn't have been able to penetrate Sasaki's closed space. Through a subsequent confrontation with the elder Mikuru Asahina, Tachibana learned of what Fujiwara planned to do with Haruhi Suzumiya's powers.

After Fujiwara's supposed defeat, Koizumi comforts a depressed Tachibana and they exchanged phone numbers and emails in order to keep in touch with esper-related issues. In "Tsuruya's Challenge", Koizumi says that Tachibana's organization has given up on trying to give Sasaki Haruhi Suzumiya's powers and on espionage activities in general.

Trivia

 * The name Kyoko means "capital city" (京) (kyou) and "child" (子) (ko).
 * The name Tachibana means "orange, tangerine" (橘).
 * It is also the name of two famous clans in Japanese history.