No problem!
First off, ⑨'s real identity is Cirno, an ice fairy that originated in Touhou Koumakyou (aka The Embodiment of Scarlet Devil), for those who don't know. She's known for being a baka, which is where the nickname originates: In Touhou Kaeidzuka (Phantasmagoria of Flower View), the manual did a typical legends job where it places a number near something to explain it:
Notably, the Japanese next to Cirno's indicator, the ⑨, translates into "Moron". This is the origin of Cirno's nickname of ⑨.
Fandom (as huge as it is), also sometimes calls Reiuji Utsuho from Subterranean Animism "6" due to being another baka and because she's the boss of Stage 6. Like with this picture, she's called a "birdbrain" by another character in game.
All info is stuff from the Touhou wikia. Not sure if the "Moron" translates right: I know little about Japanese characters.
