Elminster is an infamous ancient wizard who's been around forever, is known to meddle in local affairs all over the Heartlands and beyond, and "is something to do with the Harpers." He has a long white beard, is gaunt of frame, has a hawk-like beak of a nose, blue-gray eyes, and is often dry, sarcastic, or whimsical in his behavior. At various times he sports a staff, the Lion Sword, and a floating, Eversmoking Pipe.
Born Elminster Aumar in the now-vanished Sword Coast kingdom of Athalantar over 1200 years ago, Elminster is actually a founding member of the Harpers and one of the oldest surviving and most powerful Chosen of Mystra, meaning he both serves the goddess of magic and carries some of her silver fire within him. Variously known as "the Sage of Shadowdale" (he often dwells in an old, leaning stone tower in the dale of Shadowdale), "the Old Mage," and by many less polite names, Elminster has lead a long and colorful life, and is usually "up to" about two dozen "somethings" at any one time.
You can read about Elminster's adventures in many novels by Ed Greenwood, published by Wizards of the Coast (chronologically, the books begin with Elminster: The Making Of A Mage, and the most recent Elminster novel is The Herald, the sixth and last book of The Sundering.)