Not much is revealed about Kyon's fifth-grade sister; not even her real name is given. Other characters simply refer to the eleven-year-old as Kyon's (Younger) Sister or Little Sis. She has relatively few speaking lines, though she does appear in The Adventures of Mikuru Asahina - Episode 00, Episode 6, and Episode 14, fulfilling the duties of an irrepressible younger sister. She has even fewer lines and appearances in the original novel version than the anime's rendition. Usually, her presence is made apparent whenever Kyon is at home.

She is carefree, happy and playful and is also the force behind the use of the nickname "Kyon"; after she heard their aunt call her older brother such, she started using the nickname as well in front of his school friends. From there, the name spread and eventually stuck. She seems to get along well with all of the SOS Brigade, especially with Mikuru, sharing a sister-like relationship with her.

Source: Wikipedia