In the Phantasmagoria of Flower View manual, there was a picture showing the in-game screen, with useful notes like (1)-Player character, (2)-life bar, etc. Cirno (the strongest) was referred to as (9)-Baka
Hence, Cirno was known as (9), circle-nine (Jap: Marukyuu). And that makes 09/09-09 her day.
Fun fact: 9:09 AM today, Cirno beat Chuck Norris.