Until a staff answers this, perhaps I could provide a partial answer.

It strongly depends on the character you're thinking of making and how often they appear, or if they only appear once. For instance, the bouncer for the Equalist rally, he is only seen twice and in the same episode. For that, I don't think you would have any problem making a background for them. But a character like the LT., then making a background for him might be acceptable as we most likely won't see him next season. Bigger characters, of which are big to the plot, I would play it safe and not make a background for them.

I play Pema, of which we don't know much about her own family or how she came to be who she is, so I stated that the show didn't provide that information and in posts, I steer clear from subjects like that.

Airi's pretty spot on. You're afforded a fair amount of freedom with minor, background characters. For example, were you to take a Fire Ferret Cosplayer, it's likely that they will only appear during the probending matches, or events involving the Fire Ferrets — therefore, any backstory information you create will likely not contradict anything canon dictates about them.

No-name characters with larger roles, for instance, like the Lieutenant, may be harder to create detailed histories for because they have a larger, plot-affecting purpose in canon.

If you decide to take a canon with an unknown / mostly unknown backstory, we ask you to keep their overall canon involvement, and any reasonable expectation of future involvement in mind when creating a history for them. We have an "Anything Else" (or maybe it's "other") section on the canon ap itself — you can use this section to explain your decisions regarding a minor character's backstory.

I hope this helps!

I appreciate the insight. I had developed this whole backstory and then I was like, "hold up". But thanks for the clarification, I'll have a profile up soon.