Game concept
Character
The game also links it as Characters, Character's, Characters'.
From the game's encyclopedia
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.
Character is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Characters are either Noble or Lowborn, Landed or Unlanded, each with their own set of Skills and Traits, goals and aspirations." The game also links it as Characters, Character's and Characters'. 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.