Which DotA designer was recruited to work on League of Legends?

The DotA designer recruited to work on League of Legends was Steve Feak, better known by his alias Guinsoo.

Feak was a major developer of DotA: Allstars, the influential Warcraft III custom map that helped establish the multiplayer online battle arena genre. After working on the map, he joined Riot Games during League of Legends’ early development. His experience helped Riot build a standalone game inspired by the ideas and competitive structure of DotA.

Feak was not the same person as IceFrog. IceFrog took over DotA: Allstars development after Feak’s period as lead designer and later joined Valve to work on Dota 2. Rob Pardo was a Blizzard executive, while Tom Cadwell became an important Riot designer but was not the DotA creator identified in this question.

Feak’s influence also survives inside League. “Guinsoo” became associated with Guinsoo’s Rageblade, an item named after his development alias. Riot’s founders recruited both Feak and Steve Mescon, who ran the DotA community website.