|Wonderful Shrine Maiden of Paradise|
Her main ability allows her to fly in the sky.
Basically, she is a shrine maiden. The shrine maiden of Hakurei Shrine, which is located on the boundary of Gensokyo.
The Hakurei Shrine exists in both Gensokyo and the world of humans at the same time.
The boundary is the home territory of the Hakurei Shrine. From both sides, it is located at a very remote place from human villages, and is also kind of small, and has no known history or benefits. So it's no wonder that it rarely gets any visitors.
Inside the shrine, almost all foreign boundaries will be nullified. That alone should say that the Hakurei Boundary, the boundary between Gensokyo and the world of humans, is amazingly powerful.
Only members of the Hakurei clan have rules to follow in Gensokyo. Therefore, Reimu always wears clothes that is apparently some kind of uniform (she is supposed to have many other rules too, though...). Her daily routine consists of wearing red-white, staring at Gensokyo's sky from the edge of Gensokyo, and drinking tea.
Originally, Reimu had several various mysterious powers, but she neither uses them nor boasts about them. It's kind of a laid-back attitude.