Game concept
Mentor
The game also links it as Mentors, Mentor's, Mentors'.
From the game's encyclopedia
Mentors are Characters that help to improve the Skills of their Student. They can be gained through events when your Lifestyle Focus is set to a particular Skill.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Mentor is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Mentors are Characters that help to improve the Skills of their Student." It is part of Special Relation. The game also links it as Mentors, Mentor's and Mentors'. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.