Tsuruya is the hyperactive friend of Mikuru (from the current time plane) notable for her loud voice, ankle-length hair, and prominent tooth. She possesses a cheerful and friendly disposition. She is also prone to long fits of unrestrained and intense laughter at things she considers even the slightest bit humorous.
Her fang-like tooth gives her an idiosyncratic speech pattern, causing her to misspeak words by leaving out syllables, as seen in episode 9. Her speech pattern is often accompanied with a lisp and her catch phrases "megas" and "nyoro." Her knowledge of the SOS Brigade comes almost exclusively from Mikuru. She is also the heir of the Tsuruya family, which purportedly collaborates with Itsuki's Agency. It is also unknown if Tsuruya has a secret identity or agenda.
Though not an actual member of the SOS Brigade, she does play a minor role in Mikuru Asahina's Adventure - Episode 00 and serves on their baseball team in episode four. She also has a small appearance in the ninth and twelfth episodes of the anime, but otherwise appears relatively sparingly throughout the anime. She appears a few more times during the novels, appearing more prominently than in the anime's rendition of the same stories, and even getting along very well with Haruhi. Kyon believes this is because both Haruhi and Tsuruya are nearly insane. Though she is not a brigade member, Tsuruya is still used as an extra body for some of the Brigade's activities.
In the light novels, it is strongly implied that Tsuruya's wealthy and venerable family is a covert supporter of "the Agency" to which Itsuki Koizumi belongs. Tsuruya herself seems to possess some knowledge of the secrets of the SOS Brigade and other matters, although she never explicitly says so. Tsuruya temporarily takes care of Mikuru Asahina during an event when the latter has a double presence in the "present" of the SOS Brigade and is also the caretaker of a mysterious artifact discovered on her family's grounds, both at Kyon's request.
In the original light novel The Disappearance of Haruhi Suzumiya, Tsuruya's role in the altered reality is that of Mikuru Asahina's best friend. She is shown to be extremely protective of her, even resorting to violence to keep her safe, but still maintains her cheerful disposition.