Rules on fanon and canon:

There's really onle one master rule: Be consistent.

Whether you choose to play canon or fanon, the admins will request that you submit the name of the work you think will be closest to your portrayal of the character, and this should be part of your bio under the appropriate field. Some fanons will not be allowed on the basis that it works poorly in the system (Hang in There! Kogasa-san's Kogasa Tatara, for example, will not be allowed due to it being a self-insert of the author.).

Expect your favorite fanon to clash with certain canon situations, and sometimes other characters. The most famose example would be Momiji Inubashiri and Aya Shameimaru's fandom relationship changing between Mountain of Faith and Double Spoiler. If the Aya player picks a fanon where Aya and Momiji has friendly relationship, and the Momiji player picks the canon where they have a somewhat unfriendly relationship, this will need to be handled on case-by-case situation. This will extend to OC relationships, whether the OC be from a Touhou fanon or your own creation.

The judgement for the relationship is simple: The two players owning the character talk this out between themselves until they find something they agree with. If they fail to reach an agreement, the older player has the final say WITH THE EXCEPTION where the newer player pair appeals to the admins with good enough a reason.

While the system isn't implemented, I would like to request a character-by-character relationship memo/note ICly. Like, within +info. For example, a section called "+info/relationship/Aya Shameimaru" for Momiji.