Summerscales has been Naked's head of strategy for the past three years, and most recently set up a Naked office in Paris.
The division, launching this summer, will offer the creation of branded content, events, brochures and digital solutions. It will be based in W&K's Shoreditch offices. Summerscales will recruit a small team but will also use W&K's planners.
Fat will initially work with Honda, a W&K client that Summerscales has worked with at Naked for the past two-and-a-half years. But the division will also work independently, with outside clients.
The initiative is being led from London by W&K's managing director, Neil Christie (pictured), but if it is successful, it is likely to be rolled out elsewhere in the network.
Christie said: "We are delighted to have Simon on board. With the launch of WK Fat the agency is embarking on a very exciting journey, one which will allow us to seek out new ways of building strong and provocative relationships between our clients' brands and their consumers."