Steve Feak, also known by the nickname of Guinsoo, is an American game designer, and was a previous designer of the popular Warcraft III: The Frozen Throne custom map Dota Allstars, after Meian and Ragn0r and before Neichus and then IceFrog. Currently he is employed by Riot Games in the making of League of Legends, an action real-time strategy videogame inspired by Defense of the Ancients.
The original Defense of the Ancients map created by Eul for Warcraft III: Reign of Chaos was never ported to The Frozen Throne. So many mapmakers created their version for the Warcraft III expansion in the wake of Eul's map, using his three lane layout as a basis. Then a mapmaker known as Meian created his own version of DotA in which he gathered some of the best heroes from all the other maps. This version was called "Allstars" for this reason, and soon after its release it become the most popular version of DotA.
The development of the map was then passed over to Ragn0r, and after he left the development, then Steve "Guinsoo" Feak became the mantainer of that map. He added a recipe system for items so that player's equipment would scale as they grew more powerful, as well as a powerful boss character called Roshan (named after his bowling ball). Feak said when he began developing Allstars he had no idea how popular the game would eventually become.
He also has an item named after him in Dota Allstars called Guinsoo's Scythe Of Vyse that originally was an upgrade of Eul's Scepter of Divinity, just like Allstars was seen as an upgrade to the original DotA. In Dota 2 this reference was removed for copyright reasons (there's a Guinsoo's Rageblade in League of Legends).