Today I found this post by /u/pk_powered in the /r/Touhou subreddit about what alignment Touhou characters might be, and the linked spreadsheet also includes a column suggesting classes for the characters.
A comment by /u/Callinectes mentions taking levels in fey, outsider, and aberration (not all at the same time):
Now, let's see. You've given D&D (presumably 3.5e rather than 4e or 5e) classes and Pathfinder classes as a category. I can definitely help with that. Going down the line:
Rumia probably doesn't have class levels, but she'd be a Commoner or just take levels of Outsider (Edit: Or Aberration, depending on how you see Youkai) if she did.
Letty Whiterock is probably just a Fey.
Kogasa Tatara is just an Aberration with Aberration levels.
Are there any alternate rules I'm not aware of that make this possible? Or are they just referring to monsters as PCs rules from the SRD?
If this is possible, it is something I want to do.
We have multiple races based on monsters, and the DM allows 30 RP if we decide to make our own race. I asked my DM if I "can make a race that gets no class only racials as it levels - basically a monster that levels up" and he said "no for now".
I’m also pretty sure "too ridiculous" isn't in my DM's dictionary. At one point we had a player who was a talking Deku Nut, a player who was a magical girl idol bard, and we even have a scenario called "the Powerpuff Girls apocalypse". As long as I can avoid making it OP and justify its existence, he’ll let me get away with it.
Also, on that note, ignore any balancing unless you can somehow balance for a game where you can deal 306 damage in 1 turn and not kill the intended target outright. At level 6, no less.