She's the current "Child of Miare," a title that passes through the Hieda family once every few centuries to someone born with perfect visual memory—the ability to record and never forget anything they see. Her job is basically to document Gensokyo's entire history.
What's wild is that despite being young, her memories span back 1200 years. That's because each Child of Miare is a reincarnation of the last one, inheriting all their memories. She's the ninth in the line. Only she and Yukari seem to know about this arrangement, though she's dropped hints about it to Reimu. There's a catch though—with each reincarnation, the lifespan gets shorter.
The Hieda family has been working on a massive written history of Gensokyo for generations, called the Gensokyo Chronicle. Perfect Memento in Strict Sense, which she's been compiling, is basically her version of it. Also, she apparently has a taste for red tea.
Content compiled by AnimeList.moe from publicly available sources.
