A female friend of Kyon's from middle school. She first appears in volume nine.

She introduces herself as a "close friend" of Kyon's, evoking feelings of jealousy in Haruhi. Many of Kyon's middle school friends, including Kunikida and Nakagawa, had developed the mistaken impression that Kyon and Sasaki were boyfriend and girlfriend, when they had simply become good friends due to attending the same cram and middle schools. It was this perceived relationship which led to Kyon getting the reputation of being interested in 'weird girls'; Sasaki is prone to odd philosophical musings and behavior which some might perceive as being atypical. For instance, she would use the male 'boku' for 'I' when speaking with boys. Sasaki becomes a source of interest not only because her presence causes Haruhi to manifest Closed Space, but also she has attracted the attention of a rival Agency similar to Koizumi's, who believe that Sasaki is the true god of the world and Haruhi a mere imposer. She is also capable of creating a distinctly different form of Closed Space. While Sasaki is, like Haruhi, pretty and intelligent, there are considerable differences between them. Sasaki displays none of Haruhi's boundless and sometimes random energy and seems to be somewhat dismissive of all emotion, considering it an inhibition to human development. Both Sasaki and Haruhi claim that emotions of love are like a mental illness. Haruhi, however, has admitted to sometimes feeling a desire to engage in a romantic relationship and is known to have dated rather a lot. Sasaki apparently has no desire to do so. Also like Haruhi, Sasaki has been in contact with an alien, a time traveler, and an esper. Unlike Haruhi, Sasaki is somewhat aware of the backgrounds of her companions, although she feels that she does not understand the specifics as well as Kyon does.

(Source: Wikipedia)