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 Merriman is the oldest of the Old Ones. He was once known as "Mer-lyon" or "Merlin", chief advisor to King Arthur. He kept from Arthur the secret of his son, Bran, whom he smuggled into the future with the dog, Cafall. Guinevere 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. Merry 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 known to travel the world during the course of the sequence, usually under the cover of his Oxford professor persona. In Britain, he appears in Wales, Cornwall, Oxford, the Burnham/Windsor area, and at the archaeological site of a Roman fort. He also makes excursions to Greece, Jamaica(carrying out a government survey), and America(lecturing at Yale in connecticut, then staying in Ohio).
Merriman is the only character in all five books.