§ 127.1 Core Rules PDF (officieel)

For gameplay purposes, a card's Owner is the player who brought it into the game, either as their Champion Legend, one of their Battlefields, as part of their Main Deck or Rune Deck, or Created during the process of play. The legal owner of a card doesn't matter during gameplay. Example: Bo didn't bring a deck, so Alice loans him a deck. During the game, Bo is considered the owner of those cards, even though legally they belong to Alice.