Summarized rulings on what characters are allowed for NSFW, based on multiple staff discussions:
Any character that is stated in-game or in official mihoyo media to be underage is NOT allowed to have NSFW (note: non-mihoyo webpages claiming ages for characters are not a valid source - it must be an official mihoyo statement or something mentioned in-game).
For any character without an officially stated age, the decision to allow/reject is based on model type. Any character using the medium or tall model has NSFW allowed; any character using the short model is NOT allowed (and this includes characters using the short model that are stated to be over 18 such as Nahida). Here would be a full list that can be sorted by model type: https://genshin-impact.fandom.com/wiki/Character/List . The rationale behind this decision is that almost every character in genshin uses one of 3 nearly identical base models so in the absence of further information we moderate based on model type instead.
Any characters that fall outside of the 3 base models (such as NPCs) will be handled on a case-by-case basis. In addition, in any cases where a mod has visibly aged down characters from their original appearance may also be removed.
Submissions that violate our Indecent Content Rules or Content Visibility Ratings policy will be removed or withheld pending the removal of the violating content, or the appropriate rating applied. This covers mandatory ratings, forbidden scenarios, and inappropriate use of the content rating system.
Mod sexualizes a character with the child body type.
Mark your submission with the appropriate Content Visibility Ratings, or remove the content that violates the Indecent Content Rules. Remove any content ratings that do not apply to your submission's content.