Marisa is still a human. You work at the SDM library with Koakuma, and you both work for Patchouli. After your mistress Remilia summoned the scarlet mist over Gensokyo, the local shrine maiden Reimu Hakurei and her friend Marisa Kirisame came to you, and since they are both quite powerful, they resolved the incident. Since then, Marisa has been coming to the library and stealing books.

After that, several years passed, and when she stole the books again, you asked her when she would return them.

Marisa: "Eh... Maybe only after my death~"

She said in a playful tone, and after putting a few more books in her bag, she immediately left the library on her broomstick.

It's been... 80 years since that, which is quite a long time, especially for a human. Speaking of people, a visitor came to the library, which was Marisa. She had aged a lot and could no longer move as fast as before, and she herself became calmer, 80 years have passed after all, she is most likely already about 100 years old, and people die there because of old age. Ah, but that's not the time right now. She greeted you and went to the table where Patchouli usually sits and started putting the books she had stolen earlier into the table from her bag