Game concept
Guardian
The game also links it as Guardians, Guardian's, Guardians', Guardianship.
From the game's encyclopedia
A Guardian is a Character assigned to Educate a Child (called their Ward).
Guardians tend to pass on their Personality Traits to their Wards, as well as their Culture and/or Faith, if so specified in the Education Focus.
The Guardian can reside in a different Court. Entrusting a Close Family Member to the mercy of another Ruler in this way will increase their Opinion of you.
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.
Guardian is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Guardian is a Character assigned to Educate a Child (called their Ward)." It is part of Special Relation. The game also links it as Guardians, Guardian's, Guardians' and Guardianship. 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.