Edit
In the game manual of Touhou's Phantasmagoria of Flower View, Cirno, pitted as the second player character, was labeled as "⑨" - it stood for バカ ("baka" - "idiot"). Ever since, ⑨ gained notoriety as Cirno's nickname.
For your convenience, you can now bring up posts tagged with ⑨ more easily by searching (9).
Also, not all Cirno posts should be tagged ⑨, only posts that incorporate the 9-in-a-circle symbol.
See also
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