Merriman Lyon or "Great-Uncle Merry" is an important character in the series, and one of the most powerful and important allies of the Light. While generally very kindly and encouraging to his young friends, he can also be quite strict and forbidding. He has a mysterious past. Merriman is the only character who appears in all five books.
Warning: this section contains spoilers.
What is known is that Merriman is the oldest of the Old Ones; it is not clear how old that makes him. It is clear, however, that Old Ones have been associated with the very earliest annals of human history (given that there is a Sign for each age). However, it's impossible to say exactly for how much of human history Old Ones were actually contemporaries of their human brethren, and for how much of it they simply used their time-travelling powers to visit older times.
Though he can travel through space as well as time, he seems to focus his efforts on Great Britain.
He has lived in many different ages under many different identities. In the fifth book, he is known in Roman Britain as a druid. He was also 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. Merriman was also a lord during the Middle Ages, where he adopted the boy Hawkin, who eventually came to betray the Light. In modern times, he is a professor at Oxford going by the name Merriman Lyon. In this capacity, Merriman is known to travel the world. 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).
After the Dark is turned back for the last time, he leaves the mortal world to find his eternal rest with Arthur, the Lady and all of the Old Ones except Will.
Role in the books
Merriman works tirelessly for the Light, guiding the Old Ones, helping Arthur and other such mortal allies. He acts as a guide for the Drew children, Jane, Barney, and Simon, helping them as they try to fulfill their prophesied destinies regarding helping the Light. He acts as the mentor and teacher for Will Stanton, the last of the Old Ones, and prepares Bran for the upcoming conflict.
He is a member of the Six who will stand against the Dark in the final rising.
Merry has developed many friendships, including one with Taliesin/Gwion and with the Old Ones and other allies of the Light.
He also frequently befriends mortals, although this is usually because they are to play an important role in the struggle against the Dark. For example, he befriends the Drew children's grandfather, becoming an integral part of the family, because it has been long prophesied that the children will be part of the Six. He also befriends Will's aunt and uncle so that he can influence them to invite Will to Cornwall in the third book.
Although he does care deeply about people, Merriman can be, like the Light itself, so singly dedicated to the goal of eradicating the Dark that he puts people second. In the third book, when Will states that the Drew children will not like him, Merriman tells him that does not matter; only helping the Light matters. This sentiment caused a great deal of trouble in the 19th century, as outlined in the second book, when Merriman used his mortal liege-man and adoped son, Hawkin, as part of a dangerous spell that protected the Book of Gramarye. When Hawkin realized that his adopted father was willing to risk his life for the cause, he betrayed the Light.
Merriman is described as very tall and skinny, with a rather forbidding appearance: a shock of wild white hair, deepset serious eyes, and a curving nose. His hawk-like nose may be the reason that he is referred to as a hawk, specifically by Arthur. This, in turn, may be the reason that his liege-man and adopted son from the Middle Ages is known as Hawkin.