Elminster Aumar, also known as the Sage of Shadowdale, is one of the most famous and powerful wizards in all of Faerûn, a fair fighter and thief, and a founding member of the Harpers.
The player first meets him after 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 also makes a short appearance in Baldur's Gate II: Throne of Bhaal, where he appears near the entrance of either Abazigal or Sendai, depending on which one is the fourth to fall to the might of Gorion's Ward.
An illusion of Elminster, created by two imps for their riddle game, appears in the Sahuagin City.
Elminster article at the Forgotten Realms Wiki, a wiki for the medieval fantasy universe named the Forgotten Realms.