The basic structure of most mythological or fantasy stories retains certain elements essential to the hero's journey. These elements are retained despite slight variances. To that end, the story of Percy Jackson and The Olympians: The Lightning Thief is no exception.
Chapter One through Four chronicles the beginning of Percy's call to adventure. The Minotaur's pursuit of and subsequent attempt to kill Percy is the catalyst for an important change in the direction of Percy's life. The Minotaur's attack signals change due to an external source of pressure; this event begins the protagonist's journey into the beginnings of his quest.
CROSSING THE THRESHOLD. At the end of Act One, the hero commits to leaving the Ordinary World and entering a new region or condition with unfamiliar rules and values
THE CALL TO ADVENTURE. Something shakes up the situation, either from external pressures or from something rising up from deep within, so the hero must face the beginnings of change.
Chapter Five begins the hero's induction into a world different from the one he has come from. In Chapter Eight, the hero begins training for his future quest. This training commences under the watchful eye of the camp director, Chiron.
MEETING WITH THE MENTOR. The hero comes across a seasoned traveler of the worlds who gives him or her training, equipment, or advice that will help on the journey. Or the hero reaches within to a source of courage and wisdom.
In Percy's previous human world, Chiron is disguised as Mr. Brunner, a Latin teacher. In the hero's new world, Chiron is actually a mythological Centaur, and the wisest and most honored of the Centaur race. He is a teacher and mentor of heroes.
Among his students, Asklepios perfects the art of healing and becomes known for his ability to bring the dead back to life. Jason, another famous student, brings the Golden Fleece back from Colchis. Achilles is also another of Chiron's famous students. At Half-Blood Camp, Percy meets other half-blood demigods like himself. All of the campers are children borne from unions between immortal gods and humans. Chapter Eight introduces new allies for Percy (such as Annabeth, daughter of Athena, the goddess of wisdom and war) and solidifies the hero's present allegiances (such as Grover, the satyr). Grover, the satyr, continues his role as Percy's confidant. He confesses that his quest runs parallel with Percy's and that their 'fates were still tied together.' In fact, he has been assigned to protect the son of Poseidon on his future quest (Percy finds out his father is the God of the Sea towards the end of Chapter Eight).
It is Grover who informs Percy about the pact the Big Three (Zeus, Poseidon, and Hades) make at the end of World War II. All three swear on the River Styx that they would no longer have affairs with mortal women, as the progeny from these unions have caused all the carnage and wars on Earth.
TESTS, ALLIES AND ENEMIES. The hero is tested and sorts out allegiances in the Special World.
So, in Chapter Eight, both Chiron and Grover stand in for mentors. While Chiron is the undisputed leader everyone looks up to, Grover serves as Percy's ally and protector. Both are necessary in the evolution of Percy's transformation from normal kid to hero. In this chapter, Chiron sets the standard for the battle training game, 'Capture The Flag':
"You know the rules. The creek is the boundary line. The entire forest is fair game. All magic items are allowed. The banner must be prominently displayed, and have no more than two guards. Prisoners may be disarmed, but may not be bound or gagged. No killing or maiming is allowed. I will serve as referee and battlefield medic. Arm yourselves!"
To summarize, this meeting of mentors and the solidifying of allegiances in Chapter Eight chronicles sections of the hero's journey for Percy. For more information about the hero's journey, please refer to the links above and below.