My best guess would be to make her a tiefling expert able to use comprehend languages 1/day instead of darkness.
While ZUN says she likes pranks, she's depicted in fanon as rather moe. Combine that with her being Patchouli's assistant and I'd say she have a Neutral alignment.
As for stats, the only thing I can tell for sure is to focus on Intelligence.
For Skills, I'd focus on whatever Knowledge skills would be appropriate for a librarian along with Craft (books) and Profession (librarian), as well as Perception to deal with Marisa.
Better late than never, right?