Merriman Lyon or "Great-Uncle Merry" is an older friend of the Drew family. While generally very kindly and encouraging to his young friends, he can also be quite strict and forbidding. He has a mysterious past.
What is known is that He is the oldest of the old ones. He was once known as "Mer-lyon" or "Merlin", chief advisor to King Arthur. He did keep from Arthur the secret of His son Bran, whom he smuggled to the future with the dog cafal. Gueneviere was afraid that her husband would believe the child was Lancelot's, given her prior indiscretions. Merriman wanted to place the boy in the right time for the second great rising of the Dark.
He acts as a guide for the Drew children Jane, Barney, and Simon. He acts as the mentor and teacher for Will Stanton. He has developed many friendships, including one with Taliesin/Gwion. His profession in the present is that of a professor at Oxford, but one winter he fills in as a manservant for Miss Greythorne, when her butler is on holiday.
Merriman is the only character in all five books.