Childhood's End originally began as a short story called "Guardian Angel." The story basically consisted of the first half of the novel, until the point at which the Overlords reveal themselves. The original concept behind the Overlords, in the short story, was not that they were shepherds for the human race. The point of "Guardian Angel" was simply the irony of having these beneficial aliens turn out to look exactly like our conception of the devil. Only later, when Arthur C. Clarke was pushed by his publisher to expand "Guardian Angel" into the full novel that would become Childhood's End, did Clarke introduce the idea of the Overlords as shepherds watching over the next step in human evolution.
There are two ironic conceits in "Guardian Angel." The first is that Clarke takes the concept of an alien invasion, already a cliché in 1953, and turns it on its head: the Overlords come down in their ships over all the major cities and, instead of blowing them up, they turn Earth into a utopia. The second conceit is that these friendly aliens look exactly like a medieval conception of Satan. "Guardian Angel" was about challenging preconceived notions and the nature of prejudice, and the Overlords are the main symbol of that theme.
In Childhood's End, the Overlords become much more than ironic symbols. They become tragic figures, forever stagnant, helplessly subservient to a transcendental force much more powerful than themselves. They must watch time and again while other races achieve the next step, crossing the galaxy and even the universe with near-omnipotent powers, while the Overlords continue to zip around in their spaceships. Childhood's End is perhaps one of the most ironic science fiction novels ever written, and there is no better symbol of that irony than the Overlords, who at first seem so radically superior to humanity, but truthfully envy it.
Karellen is the only character to appear throughout the entire novel. He is the leader of the Overlords on Earth, and is known to humans as the "Supervisor." At first, Karellen only interacts with the Secretary-General of the United Nations, Stormgren, but after a generation he reveals himself and his fellow Overlords to mankind. Karellen's attitude toward humans is never one of superiority or contempt, despite the Overlords' immense technological and intellectual achievements. Karellen (and his fellow Overlords) actually seem to view the humans with pity. They can identify with humans such as Stormgren and George Greggson, humans who are still flesh-and-blood creatures like themselves. But Karellen also knows that humanity will eventually be utterly destroyed when this next step is taken, as a generation of human children suddenly become creatures of pure intelligent energy, part of an "Overmind" that criss-crosses the galaxy looking to add more races to its own form. There is something horrible in this, particularly in the way the rest of human society destroys itself while awaiting its children's bizarre armageddon. Karellen is trapped in his role as a supervisor, and it is his job to nudge the human race in the right direction until it reaches the necessary developmental stage. His motivations are simple: he has no choice but to obey the Overmind or presumably suffer some kind of consequences. But Karellen would do the job anyway, if only for the opportunity to examine the process and, perhaps, discover the secret to it.
Of all the characters in Childhood's End, George Greggson, in his simple yet insightful observation of the events of the twenty-first century, probably comes the closest to being a protagonist. It is George's own children who begin the unstoppable processes of change that eventually lead to the transformation of mankind into a part of an intergalactic "Overmind." But George lacks an important aspect of being a protagonist: his character is not particularly dynamic. Indeed, his role is a very passive one. He is a witness to the fulfillment of the Overlords' plans, watching his own children change into something much greater than humanity has ever produced. George is an observer, and his role is to give the reader an ordinary man's perspective on this final period of human history.
Jan Rodricks represents what is lost in the great "evolutionary" changes that are wrought when the children of humanity join the Overmind. When humanity is still enjoying its utopia under the auspices of the Overlords, Jan becomes dissatisfied with life. He wants to explore the stars, and to visit planets as the Overlords do. Jan represents humanity's inability to ever be satisfied, to ever be truly content, or have its curiosity sated. It is admirable that Jan rejects the blandness and boredom of utopia in favor of taking risks (i.e., sneaking onto the Overlords' ship). It seems likely that there would be many more men and women like Jan in a world such as the Overlords'. Satisfaction in life, no matter how good, has never been one of humanity's strong suits. Jan is a symbol of that truth, and it is fitting that he is the last witness to the destruction and transfiguration of mankind.
Of all the "characters" in Childhood's End, the Overmind is perhaps the most difficult to understand. The reader is given only a little information about it. The Overmind is sufficiently powerful to make the Overlords their permanent servants. When the children of Earth join the Overmind, they utterly destroy Earth in the process, using its matter to fuel their transformation. The Overmind's motives and its mission are completely unknown; Karellen's theory is that it is "trying to increase its awareness of the universe," i.e. learn as much as possible. Races that join the Overmind lose all their individuality. Most interesting of all, despite the Overmind's apparent omnipotence, races such as humans can serve as a threat to it if they scientifically study phenomena such as telepathy and extra-sensory perception. At the conclusion of Childhood's End, the Earth is destroyed, all adult humans are dead, and the evolved children of humanity have lost themselves in the collective conscious of the Overmind. This seems to be nothing less than a racial Armageddon. It is difficult to agree with Karellen's assessment that this event is "wonderful." There appear to be only two options: racial stagnation, like the Overlords, or losing one's race's individuality in the Overmind. But there is a third option—to study ESP scientifically in a race capable of it, without joining the Overmind. This, however, is what the Overlords are sent to prevent.