Cirno is Bolt's mascot of choice, albeit she wasn't normally chosen to be the mascot in question - but was supposed to embody Bolt anyway (at least, in the early years of his involvement in IaLR). She is controlled by BoltBlizard.
Cirno's obvious abilities come from her being an ice fairy - she can freeze people, alter said ice, and fly all over the place. Those are her main abilities. It's worth noting the temperature of her ice in what it can freeze, stop and block. In particular, she can freeze practically anything (as long as it's not of a fire-like origin) and block just about anything else (most notably being able to use ice to block lasers).
Cirno is... a weird individual. If one thing is certain - expect anything, for Cirno is relatively unpredictable. When her mind is set to something, expect her to follow through with it and not cease until that which she saw to is all good and over. However, she tends to wind herself in a lot more trouble than she can reasonably handle with her power, thanks to her high self-confidence and being something of an idiot.