Merlin

Merlin is first mentioned and introduced in the novel "The Legacy of Merlin" as Prue and Piper go back in time to meet him. He is then mentioned briefly in "We're Off to See the Wizard" and "Sword And The City."

Description according to The Legacy of Merlin

 * "A tall, gray-beared figure in a long blue robe stood above her on the stairs. Iron gray hair waved back from a high forehead. Deep-set eyes burned down at her from shaggy brows."
 * ~ Piper (Chapter 10)

About Merlin
In The Legacy of Merlin, he is presented as the most powerful Druids of all time. He is said to be the advisor to King Arthur. He lives in a high tower, protected by magic, in Camelot which can be accessed from the castle's hall. It is revealed that he is married to Nifein the Enchantress, also sometimes known as Viviane and Nimue and that he would have a son with her, Niall. Soon after Niall was born, his wife imprisoned him with her in a castle of magical glass and Niall was adopted by a noble family close to King Arthur whose parentage was kept a secret from everyone.

In We're Off to See the Wizard, the Wizard states that Merlin is an overrated hack whereas in Sword And The City, Mordaunt said that he never existed but was only a tale.