A strong ice fairy. She is very popular in the Touhou fandom.
She is weak (in comparison to other characters, but strong for a fairy). She says "I'm the strongest" in PoFV a few times.
The general consensus is that she is an idiot, because an official Touhou manual, explaining the layout of the screen, numbered the Cirno 9 and labelled her as an idiot.
This tag implicates touhou (learn more).