You manage a brand. You have a product to promote. You sign up a celebrity. Simple. Or, if you prefer, you invent a character to promote your product (remember The Milky Bar Kid?). Easier to control, and less risk of him or her being photographed stumbling drunk out of Whisky Mist at 2 in the morning.

Or, if you're MTV, you take this idea and subvert it by creating a virtual celebrity to promote a cause rather than a product. So, welcome Cherry Girl. She aims to "show a lifestyle that is fun and fulfilling, and, as a by-product, happens to be sustainable", according to MTV's Head of Sustainability, Ian Jackson, whilst being "mischievous, rebellious, seductive, intriguing and hedonistic". So nothing like the Milky Bar Kid, then.

Building on MTV's Switch campaign, which communicates climate change messages through a variety of media, Cherry Girl inhabits a virtual world - through her blogs, tweets, MySpace page and Facebook profile. She extols the virtues of reusing and recycling (and cycling), and debates the issue of sustainable consumption, with a cool and slightly off-beat approach. All very web 2.0, with the emphasis firmly on dialogue and user involvement.

Cherry Girl Video from Cherry Girl on Vimeo.

It feels like an inspired idea - it engages the MTV audience through channels they understand and use everyday, it does it in a way that feels inspiring, contemporary and thoughtful rather than preachy, and it aligns with existing activity that's already up and running. 2,136 Facebook friends may not be enough to change the world, but it's worth remembering that MTV's long running Staying Alive campaign, promoting safe sex messages to youth globally, reached 800 million homes - that's 64% of homes with a television. They know what they're doing, and you've got to respect them for trying.