"Little Haruhi" is a mysterious character who appears in The Promise of Haruhi Suzumiya. She looks like Haruhi Suzumiya three years ago, but is in fact a non-human creature with a low opinion of humanity.
In chapter 1, Kyon can see her shadow at the SOS Brigade clubroom. In chapter 2, he can try to follow her, but she will disappear. Itsuki Koizumi has poorer memory than Kyon of the time loops, but he believes that Little Haruhi must be found, otherwise the time loop won't end.
In some versions Koizumi disappears at chapter 5, in which case Little Haruhi doesn't appear again.
In chapter 6 Koizumi confronts Little Haruhi and, believing she is the cause of the time loop, attempts to destroy her. Kyon can choose to intervene or not. If he stops Koizumi, Little Haruhi gets scared of Koizumi. Kyon brings her to see Haruhi, who is surprised to see her seeming duplicate. Little Haruhi will happily play with Mikuru Asahina, but Koizumi still doesn't trust her.
Little Haruhi goes missing, which could result in the destruction of the school. Haruhi and Kyon manage to escape; the school is damaged but the SOS Brigade room still exists.
In chapter 7, at night, Little Haruhi has transformed into a "full-sized" copy of the real Haruhi and attacks her. Kyon has to decide which one is real; choosing the wrong answer results in a "destruction" ending, but choosing the right answer results in the "good" ending. In the former case, the Fake Haruhi will laugh at Kyon for making the wrong choice, say that she hates humans, and disdain him for making the real Haruhi sad. In the latter case, Fake Haruhi will show her true identity and says that humans are not such hateful things and aren't easily trapped. Afterward she will disappear.