Letho of Gulet is no stranger to those who’ve played the game preceding The Witcher 3: Wild Hunt. He was the main antagonist in The Witcher 2: Assassins of Kings, and the cause for most (if not all) of Geralt’s troubles in that game. Based on how you finished The Witcher 2, or the decision you made when establishing your world state in The Witcher 3: Wild Hunt during your little interview at Vizima, Letho can be alive in your game.
If you did happen to choose to let Letho live even after all that he did in the last game, you will come across him during the sidequest The Fall of the House of Reardon (picked up from Lindenvale). A woman will ask you to check out her old estate that has been overrun by monsters. Once Geralt arrives, he will find a barn full of booby traps that you can either disarm or choose to ignore. Head to the back of the barn and fix the ladder so that you can climb up and reveal Letho hiding from some enemies.