>>281244276Opera is a mix between a classic kuudere and a mild tsundere in the sense that they did pick on people they like (Kalego and Balam) and still do (Sullivan and Bachiko).
Both archetypes are historically popular as fuck. Also them being bigender (hinted at with merch)/nonbinary and being drawn as either feminine or masculine with each appearance makes them very flexible with shipping work.
Also Mafia Opera might add to the newer numbers. I think Mafia Opera might be more popular than the mainline one at this point.
In the same vein their design in Kalego Gaiden seems to be more popular than any of the others.
Last but not least Opera might qualify for rank nine. We do not know why they backed out of fighting Amu so it's still very possible that they are crazy strong and simply refuse to rank up like Sullivan. They also do not brag despite being THE almighty janitor of the series. That appeals to a lot of people. Large parts of the fandom consider them Crown material afaik.
Bonus: meow meow