She is the current "Child of Miare" (御阿礼の子), who is born in the Hieda family once in every few hundred years with the ability to "not forget anything she sees" and record Gensokyo's history. She seems to like red tea.
Despite her young age, her memory goes back 1200 years. It is because every "Child of Miare" of the Hieda family is a reincarnation of the previous "Child of Miare," who inherits the memories of the previous incarnates. She is the ninth reincarnation in the line. It seems that no one but herself and Yukari know this, though she does mention part of it to Reimu during Memorizable Gensokyo. Each time she reincarnates her life span gets shorter and shorter, and each Child of Miare works on a written record of Gensokyo's history called the Gensokyo Chronicle. Perfect Memento in Strict Sense is the edition of this record that Akyu is writing.