Game concept
Diarch
The game also links it as Diarchs, Diarch's, Diarchs'.
From the game's encyclopedia
The junior partner during Power Sharing, a Diarch helps their Liege run the Realm.
The Diarch has access to Borrowed Powers, and gain more of these as the Scales of Power tip in their favor.
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.
Diarch is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The junior partner during Power Sharing, a Diarch helps their Liege run the Realm." It is part of Power Sharing. It covers Vizier, Co-Rulership and Secretary Director. The game also links it as Diarchs, Diarch's and Diarchs'. Patch 1.20.1.2, from game/common/game_concepts/04_ep2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/04_ep2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.