Espers (超能力者（, chōnōryokusha, literally "person with special powers") in the Haruhi Suzumiya franchise are humans who possess ESP (extrasensory perception) and psychic powers. According to Itsuki Koizumi, who is an esper himself, espers had gained their powers when Haruhi made a "timequake" in year 0, three years prior to when the story starts.
Many espers belong to the 'Agency', whose purpose is to prevent Suzumiya's unstable emotions from creating closed space. This alternate dimension only appears when Suzumiya is stressed, and overlaps reality. It is populated by celestials, which could conceivably damage reality if unchecked. The espers consider it their duty to destroy the celestials.
These espers are capable of extra-sensory perception most of the time, but otherwise their powers are inaccessible unless they enter closed space or a similar dimension, where they can generate red force fields capable of destroying celestials, but even then, their powers may still be limited. These espers can "donate" powers to each other for use in emergencies.
In addition to the closed space created by Haruhi Suzumiya, espers from 'The Agency' have used their powers in a mixed space created by Haruhi Suzumiya and Sasaki, in space holding a primitive data organism, and in space created by a manifestation of Haruhi Suzumiya's antimatter-dispersing oscillations perceived as sound waves.
Rival and unaligned espers exist as well, and they have differing sets of powers. Rival espers can mentally access closed space created by Sasaki and even transmit visions of it to normal humans. According to Koizumi, these espers cannot access the closed space created by Haruhi.
The process for creating (and reverting) espers seems to vary to some extent. Members of the 'Agency' were transformed into espers in year 0 by Haruhi Suzumiya's subconscious (at least according to Itsuki Koizumi) and these espers immediately understood their powers, duties and creator. However, another human-to-esper conversion, Nakagawa, seems not have acquired any of this knowledge, and was confused by his abilities. When he saw humanoid interface Yuki Nagato, he mistook her connection to the Data Overmind, which he could perceive, as "love". Yet another esper, Kyouko Tachibana, believes espers were at least initially created by Sasaki.
Esper transformation is known to at least some factions—the 'Agency' once offered to transform Kyon into one (he refused) and Yuki Nagato converted confused Nakagawa into an ordinary human again. (It seemed to take Nagato a while to realize what Nakagawa was, but she transformed him instantly, causing enough confusion to interfere with his football game and sustain an injury.)
- Ability to sense a variety of supernatural phenomena
- Ability to sense, and usually enter, closed space
- The power to envelope themselves in a red ball of light while within closed space
- Can toss spheres of energy as projectiles within closed space