Nagato Yuki

Yuki Nagato is the third person to join the SOS Brigade. At first glance, she appears to be an introverted, taciturn bibliophile. She later reveals herself to Kyon as an alien created by the Data Overmind, and possesses supernatural powers as a result. She was sent to North High to watch over Haruhi Suzumiya, and over the course of her time there she comes to develop insights into being a person, as well as emotions (according to Kyon). She is commonly known by her surname "Nagato" (which is not unusual), as Kyon often called her by her surname, as opposed to how he almost always called Haruhi Suzumiya by her given name "Haruhi".

(Edited from the Haruhi Suzumiya wiki)