Game concept
Soulmate
The game also links it as Soulmates, Soulmate's, Soulmates'.
From the game's encyclopedia
A Soulmate is a Special Relation indicating a deep, profound, romantic love. It is a stronger Special Relation than a Lover. A Character can only have one Soulmate.
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.
Soulmate is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Soulmate is a Special Relation indicating a deep, profound, romantic love." It is part of Special Relation. The game also links it as Soulmates, Soulmate's and Soulmates'. 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.