Cirno is an ice fairy who is a low level boss and playable character in several of the Touhou games. Her power is the ability to manipulate the cold, although compared to most other characters she isn't very powerful. She is rather childish and acts without aim. Even though she is an ice fairy, she is able to remain active year round because she can project a field of cold around herself.
In the instruction manual of Phantasmagoria of Flower View, she was included in the diagram explaining the screen layout (which showed the score, player health, etc.) and listed as "9. Idiot". At one point in the game, she announces "I'm the strongest!", which has caught on with many fans.