The term ‘mentor’ is an old one. When Odysseus sailed off to the Trojan War he left his infant son in the care of his wife, Penelope, and his servant, Mentor.
No one realized (i.e. no mortal realized) how long Odysseus would be gone but as the years passed Mentor stood by his oath and taught Telemachus, by word and deed, the ways a civilized man must act. When Odysseus returned home he was much impressed with the way his son had turned out: brave, honest and respectable. All thanks to Mentor.
Even though Mentor was a slave, at least in today’s vocabulary, his name is mentioned (as a good example of manhood) considerably more often than Odysseus, his owner. The mythical times that spawned these stories were wild and barbaric. Kings and slaves were a common elements of these cultures. The civilized people were few and constantly struggling against the assaults of brute forces. The provincial rulers, like Odysseus, held their property by force and likewise, the people they ruled were subject to the civility or the cruelty dispensed by each local ruler.
Mentor was lucky to know Odysseus... but Odysseus was blessed to know Mentor.