It can honestly vary from canon to canon. We do accept characters as minor as the Ancestor characters from Homestuck (very minor, no real screen time, but enough background to extrapolate)! If Osamu herself has enough material behind her to put together an app, then you're free to post it. The Canon OC app gives far more leeway in terms of headcanon and widely accepted fanon theories behind characters.
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It can honestly vary from canon to canon. We do accept characters as minor as the Ancestor characters from Homestuck (very minor, no real screen time, but enough background to extrapolate)! If Osamu herself has enough material behind her to put together an app, then you're free to post it. The Canon OC app gives far more leeway in terms of headcanon and widely accepted fanon theories behind characters.
I hope this helps!