The Caretakers.

Writers. Adventurers. Scientists. Inventors. Artists. Scholars. All chosen by those who preceded them to take up the mantel of Caretaker of the Imaginarium Geographica, to protect the atlas and, thereby, the people, creatures, and places in both the Archipelago and the Summer Country.

The Current Caretakers

John, the Caretaker Principa / JRR Tolkien

After the grisly murder of his tutor, Stellan, John realises what all the intense studying, tutoring, educating had been about - Stellan had been training John to become the next Caretaker Principa of the Imaginarium Geographica. Retrospectively frustrated he did not take his studies as seriously as he should have, John perseveres and - along with Jack and Charles - agrees to travel to the Archipelago of Dreams with Bert, Aven, and the Indigo Dragon. John must lead a quest to find the Ring of Power,the heir of Arthur Pendragon, and protect the Imaginarium. More thoughtful, quiet, and prudent than his companions, John is initially unwilling to admit his faults to the others, particularly his early ineptness at reading the atlas; and he is mortified when he is the one to lose it during a crucial part of their first quest. However, throughout the first novel, John becomes a competent leader and Caretaker Principa, and demonstrates the finest (and not so finest) qualities of both roles throughout the rest of the series.

Jack / CS Lewis

Jack is the youngest Caretaker. Having recently returned from fighting in the first World War, Jack is barely twenty and frequently disturbed by the unsettling parallels between the war in the Summer Country and the war in the Archipelago. He falls quickly for Aven, though she never seems to be clear about whether or not she ever reciprocated his feelings, and he loses her to Artus/Bug. Being the youngest of the three, Jack is quick to catch onto any mistakes the older two Caretakers make and point them out. However - after unknowingly having his shadow stolen by the Winter King, and a casualty at the battle at Terminus that was far to analogous to one Jack suffered in the Summer Country - he matures greatly throughout the novel and the rest of the series. 

Charles / Charles Williams

Charles is the eldest Caretaker, and the more sarcastic and witty of the trio. However, he is incredibly loyal, and he learns to accept the bizarre reality of the Archipelago for what it is. He despises Magwich (whom he "affectionatly" dubbed Maggot), but has a soft spot for the badgers who accompany the companions on their journeys. In The Indigo King, Charles does not take part in the adventures - Jack and John are, instead, accompanied by Chaz, an "incarnation" of Charles. Chaz looks and sounds very much like Charles, but is simply from another timeline. However, Charles' return to the Archipelago is crucial in the fourth novel, as it deals intimately with time and space travel, his area of expertise. At the end of the fourth novel, Charles learns he isn't expected to live much longer, and therefore choses Fred - the grandson of Tummeler, the badger who assists Charles in the first novel at Terminus - to become his apprentice, the first of the new Caretakers to do so.

Past Caretakers

Bert / HG Wells

Presented as a "tatterdemalion" who first brings John, Jack, and Charles together, Bert is a different version of HG Wells. He was one third of the previous group of Caretakers, alongside JM Barrie and John's late tutor, Stellan. He is quite knowledgable of the workings of the Archipelago and owns a Dragonship, the Indigo Dragon, which is captained by his beautiful young daughter Aven. Bert is the new Caretaker's guide to the world of dreams, and their constant advisor throughout the series. He later becomes a grandfather, to Aven's son, Stephen.

JM "Jamie" Barrie

An ex-Caretaker, Jamie is the author of Peter Pan, and it is sugguested that perhaps he was inspired by the Peter Pan and his Lost Boys in the Archipelago, rather than by Peter Llewellyn-Davies and his brothers. Jamie left his post as Caretaker but still is instrumental in the events of The Search for the Red Dragon and their consequences in subsequent novels.

Other Past Caretakers

Those who appear at the Tammerlane House, where their portraits hang. 
The Tammerlane House is located on a secret island in the Archipelago.Tycho Brahe
Miguel de Cervantes
Geoffrey Chaucer
Leonardo da Vinci
Chrétien de Troyes
Daniel Defoe, spy for the Winter King
Charles Dickens
Alexandre Dumas
Johann Wolfgang Von Goethe
Jakob Grimm, spy for the Winter King
Nathaniel Hawthorne
John Irving
Johannes Kepler
Rudyard Kipling, a double agent, working for the Caretakers
Thomas Malory
Edgar Allen Poe, the "master" of the Tamerlane house
Franz Schubert
William Shakespeare
Mary Shelley, the only female chosen to be Caretaker
Percy Shelley
Edmund Spenser
Jonathan Swift
Mark Twain
Jules Verne, the Prime Caretaker