That cheerful looking fellow pictured above is Brad Muir, former Double Fine project lead on Massive Chalice and Iron Brigade. Though the photo was taken years ago Muir's smile could easily apply today, as he's just announced he'll be joining Valve later this year – though in what capacity, no one knows.
According to our interview with Muir, he's been with the company for over ten years. In addition to the titles listed above he's also worked on Psychonauts and Brutal Legend. That interview also mentions Muir's affection for Dota 2, so he's gone to the right place.
Inevitable question: will he be working on Half-Life 3? I don't know! Valve only appears willing to say what HL3 will not be at the moment.