A designer for Wizards of the Coast who became lead developer for Dungeons & Dragons 4th Edition in 2009 and co-designer of Dungeons & Dragons 5th Edition, before Monte Cook did what Monte Cook does best and quit in a huff right at the finish line. The "fighter's advocate" meant to counterbalance Cook's infamous " wizards da best, knuckleheads go home" tendencies. Used to be on a podcast with David Noonan.
Politics aside most fa/tg/uys would say he is batting .500 as 4e is generally regarded as a mistake and 5e appears to have achieved tentative acceptance on /tg/. He made a deal with Tzeentch to create the putrid Essentials line but everyone likes to pretend that dark time never happened. He was also the lead designer of The Book of Weeaboo Fightan Magic which, in what appears to be a theme for his work, produced a roughly 50/50 split of acceptance and RAGE among fa/tg/uys. This book was basically a predecessor to the 4e "martial fighter" philosophy so you can blame him for that too if you are so inclined.
His stated design philosophy is inspired by the first few editions of the game, which found expression through 5e's many "throwback" game mechanics, and the Essentials line's pared-down design. Again, take the good with the bad. He's also been writing the Unearthed Arcana articles on the official website, offering
new game mechanics and ideas whatever a Wizards employee wrote on the back of a napkin at Starbucks, plus discussions regarding the results of the official monthly surveys, making him the friendly face of 5e's new "look, we're actually giving two shits about what you think" marketing strategy. While this started out as a pretty cool thing, over time UA has morphed into more of a monthly advertisement for whatever's next in the 5e supplement pipeline and Mearls has stopped even pretending to care about things he doesn't personally fanwank over, as seen in the infamous September 2017 UA where he all but outright stated that nobody at Wizards gives a shit about the gith. Skub rages on.