Some shadow cards do something to the "defending player". Does that mean the player engaged with the enemy or the player who controls the character who is defending the attack?
EG: Player 1 is using a character with Sentinel to defend an attack of a monster engaged with Player 2, "Under the Shadow" (CORE) is the shadow card. Which player raises their threat?
who is the "defending player"?
The defending player is the player who the enemy is engaged with. So in your scenario, Player 2 would raise his/her threat.
Don't recall a source for that, but I'm sure I could come up with something official if you prefer
From p. 9 of FAQ:
Q: If a player does not declare any defenders against an
attack, is he still considered the defending player?
A: Yes, the player an enemy is attacking is considered
to be the defending player. Whether or not he declares
defenders, and whether or not any other player
declares defenders for him, does not change his status
as the defending player for the attack.