Watahashi, Yasumi

Introduced in Volume 10.

A freshman who is selected as the sixth member of the SOS Brigade. She was the only one to pass the "tests" Haruhi sets forth. She's an extremely enthusiastic member, all the members are amazed she can keep up with Haruhi's pace.

[Spoiler] She briefly joins the brigade in the tenth novel, claiming to be a first-year student at North High. She is instead a literal deus ex machina, created by Haruhi to block Sasaki's interactions with Kyon. When the crisis is resolved, she disappears from existence with Haruhi believing she was a middle school student who snuk into the brigade. [/Spoiler]