All 4 Uses

mentor
in
Here, There be Dragons by Owen
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John cabled a reply to his mentor, requested a temporary leave, dispatched a note to his wife of less than a year at their home in Oxford, explaining that he would be absent for perhaps several days, and immediately arranged passage to London.†
Chpt 1.1 *
If he had taken his responsibilities more seriously, then you"—she pointed at her father—"wouldn't be in danger, and your mentor"—pointing at John—"wouldn't be dead, and you"—again jabbing a finger at John—"would still be doing whatever it is you do."†
Chpt 2.5
"Believe me, Professor," John said, taking a seat opposite his mentor, "I wasn't expecting to see you, either."†
Chpt 4.14
The professor had always been friendly toward him, in the way that a mentor might be, but such directness, especially with such passionate encouragement, was more than unusual.†
Chpt 4.14

Definitions:

(1) (mentor)
someone who guides and advises another who is less experienced; or the act of providing such guidance
(2) (meaning too rare to warrant focus)
Much less commonly, in the ancient Greek story of The Odyssey, Mentor was a friend of Odysseus.