Elminster Aumar, also known as the Sage of Shadowdale and the Old Man, is one of the most famous and powerful wizards in all of Faerûn, as well as a fair fighter, thief and a member of the Harpers.
The player first meets him on entering Coast Way, He can also be met in Beregost in Chapter 3, at the Carnival and in Baldur's Gate in Chapter 5. He will also make a short appearance in Baldur's Gate II: Throne of Bhaal, he appears near the entrance of either Abazigal or Sendai, depending which ones is the fourth to fall to Gorion's Ward's might.
He also appears in the Sahuagin City as an illusion created by two imps for their riddle game.
Elminster article at the Forgotten Realms Wiki, a wiki for the medieval fantasy universe named the Forgotten Realms.