Pretty sure it would be the active player.
This is correct and here is the source:
(1.52) “The defending player”
When an enemy makes an attack against a player, or a character controlled by a player, that player is “the defending player” regardless of whose character is declared as a defender. Card effects, including shadow card effects, that target “the defending player” or “you” still target the player who the enemy is attacking even if another player declares one of his characters as a defender for that attack.