Final Answer:
Achilles' best friend and mentor was Patroclus.
Step-by-step explanation:
In Greek mythology, Achilles' closest companion and mentor was Patroclus. Their bond was forged on the battlefield, as Patroclus played a crucial role in the Trojan War alongside Achilles. He not only served as a loyal friend but also donned Achilles' armor to inspire the Greek troops when Achilles withdrew from the fight due to a dispute with Agamemnon.
The deep connection between Achilles and Patroclus is a central theme in Homer's "Iliad." Patroclus' death at the hands of Hector, the Trojan prince, is a pivotal moment that drives Achilles to rejoin the battle with a fierce determination for revenge.