This sounds like a really stupid question to me with an obvious answer, but, Google didn’t pop anything up, so, I figured I’d ask here and find out. I’m guessing I’m gonna be very disappointed.
On that note, if you reset from a hidden class (say, Miko) and want to go back to it, do you need to do the quest chain again or can you just select it?
as long as you did the unlock manually or used unlock voucher, it should appear on your F1 Screen.
Example:
Unlocked: Bullet Marker, etc.
If you unlock it via event, once you reset you’ll have to do the quests.
Circle costumes, what do you mean? Exo bonus costumes? BM Firecrackers? if those, you get to keep them but they won’t show up unless you are the respective class using the costume.
If you mean Miko costume without having a miko rank, they will be taken away on reset.
Say you’re an Onmyoji. You have unlocked the class via the Master’s quest. This will appear as “Unlocked: Onmyoji” on your character screen and will be unlocked forever even if you reset your character into something else. So you’ll be able to reset back into an Onmyoji at any time.
Now using a rank reset voucher deletes all your costumes from your inventory/storage. Unfortunately… this means that if you reset your Onmyoji to something else then back to Onmyoji, you will have lost all your costumes. So you’ll have to suffer through the minigame again to get them back 
Feel free to tell if I’m wrong here…
I reset my BM and I get to keep my firecrackers as explained on my initial post.
If onmyoji takes it away then it’s a different case?
You are wrong.
You keep both the costumes and the costume special items (for r8 classes). You cannot wear the costumes unless you change back to the class though.
Ok thanks. Never tried myself because I feared I would lose the costumes, now I know I can.
Do someone know if you can retry costume quest if you reset your BM for exemple if you unlocked it without the quest ? 