This is a loaded question but here it goes...
Ged/Duny/Sparrowhawk (The Hero):
Ged is "given" the power of magic by his mother. By that, I mean he was born with the gift. However, it's his aunt, the village witch, who helps him realize his powers when he accident puts a group of goats under a spell.
Ged's magic moves the story along because, when Kargish invaders come to seize his village, the boy is able to create a fog in order to hide the townsfolk.
This gets the attention of Ogion the Silent.
Ogion the Silent (The Mentor):
Ogion the Silent is a mage who takes Ged under his wing.
When Ged reads a spell from one of Ogion's books and accidentally creates a shadow creature, Ogion decides that he will send him to a school of wizardry on the island of Roke so he can learn to control his powers.
Technically, by sending him away, Ogion gives Ged the gift of education.
Archmage Nemmerle (The "Christ" Character):
Archmage Nemmerle is the Warder of Roke. When Ged first arrives, it's Archmage Nemmerle who greets him.
Archmage Nemmerle is old and wise but he's still extremely powerful.
Shortly after he's introduced, he ends up giving up his life to save Ged from the dark spirit he summons while dueling Jasper.
Therefore, Archmage Nemmerle provides Ged with the gift of life - a second chance, so to speak.
Jasper (The Foil):
Jasper is a mage two or three years older than Ged. The two do not get along and the rivalry between them causes trouble almost immediately.
At one point, the two face off in a duel (which is forbidden) and Jasper challenges Ged to summon a spirit from the dead.
Though Ged does not know how to safely summon a spirit, he attempts to do so anyway and, as a result, a dark spirit escapes the world of the dead and attacks him.
Though Archmage Nemmerle is able to ward it off, it takes all of his power to do so and, soon after, he dies.
Jasper highlights Ged's negative qualities, such as arrogance.
Pechvarry (The Man In Need):
Pechvarry is a boatmaker who befriends Ged when they meet on the jetty.
He has a young son who is ill and close to death, so Ged steps up to the plate and tries to heal him. Unfortunately, he fails and the boy dies anyway.
This experience teaches Ged that he simply can't save everyone he tries to help.
From there, he continues his journey with a heavy heart.
Skiorh (The Possessed):
Ged meets Skiorh when he travels to Osskil in order to seek advice regarding what to do about the shadow that's following him.
However, while they're traveling, the shadow overcomes Skiorh and possesses him. It attacks Ged but, somehow, the boy is able to fight it off.
He falls unconscious immediately after and eventually wakes up in the Court of Terrenon.
Serret (The Beautiful Temptress):
When Ged awakens in the Court of Terrenon, he finds Serret waiting for him.
She tells him that he is safe and tries to get him to speak to the ancient stone locked deep within the castle.
Neither she, nor her husband, can access the stone's powers, so she lies to Ged and tells him that the stone, itself, told her to have him take advantage of its forces.
Ged then realizes that the entire thing is a ruse and he transforms into a falcon and flies away.
From There To The End:
In falcon form, Ged flies back to his hometown where he seeks out Ogion.
He asks Ogion what to do about the shadow that's following him and Ogion advises him to confront it, as opposed to continuing to run from it.
Ged obliges and tracks the shadow across the sea. When he finally confronts it, he comes to the realization that it's actually just a part of his own spirit.
Once Ged's able to reconcile that knowledge, the shadow melds back into him and he becomes "free" once more.
* Please note that A Wizard of Earthsea is a very "involved" novel. Though I've done my best to outline the main characters, and how their actions work to move the plot forward, I suggest you skim the book in order to further understand the context in which they exist.