Tsuruya is fictional a character from the Haruhi Suzumiya franchise. Although not a member of Haruhi Suzumiya's SOS Brigade, the energetic girl is the best friend and classmate of Mikuru Asahina. While she is very casual and social around her friends and school mates, she is a decendant of a venerable and wealthy Japanese family.
Tsuruya is a second-year student at 'North High' high school and good friend of the time-traveling Mikuru Asahina (although she assures Tsuruya to be from our current temporal plane). Her given name is still unknown, but she is easily identified by her knee-long, green hair, loud voice and a fang-like, left eyetooth which is prominently visible whenever she bursts into unrestrained, intense laughter. She sports a very upbeat, hyperactive personality not unlike Haruhi's when she is excited and, unsurprisingly, the two get along very well, with Tsuruya at times even 'betraying' her friend Mikuru for a few good laughs. Due to her eyetooth, Tsuruya also tends to misspeak certain words, occasionally leaving out syllables, which she makes up for with an overabundant use of the word 'mega' as well as often adding the phrase 'nyoro' to the end of sentences.
Although not an official member of the SOS Brigade, Tsuruya is a welcome stand-in whenever Haruhi is in need of additional participants for her planned activities. She first joins the Brigade's baseball team on Mikuru's proposition and later plays a minor role in Haruhi's movie The Adventures of Mikuru Asahina Episode 00 (in which she ruins an entire scene by laughing uncontrollably.) She is also named an 'Honorary Consultant' by Brigade Chief Haruhi after lending her family's mountain resort to the Brigade's New Years activities.
The later novels of the franchise provide various implications that Tsuruya's venerable family is likely one of the wealthy donors of Itsuki's organization of espers called the 'Agency'. To what extent she herself knows about the secrets behind Haruhi's existence and her Brigade members' is never openly discussed, but some of Tsuruya remarks to Kyon occasionally allude to an often surprising level of comprehension of the rather unorthodox events she witnesses from the sidelines.
On a side note, Tsuruya is not to be confused with Churuya, an obvious chibi knock-off of Tsuruya starring in short, 4-cell comic strips (4koma). Churuya sports similar character traits, predominantly her love of the phrase 'nyoro' and has a worrying obsession with smoked cheese. Also known as lol-tan.