by blindpanzer
From the rules, pg. 16In all cases, the player is considered to have engaged the enemy and the enemy is considered to have engaged the player.
The enemy is engaged with the player NOT with a specific character. During the combat phase you chose which one of your characters (ally or hero) to defend and any characters you want to use for an attack.
In each round you can use whichever eligible (not exhausted) character you control to defend (or another player can use his/her sentinel character and ability to defend for you). This means that you may defend with one character in one round and with another in the following one.
From the rules pg. 20,
Any enemy cards that are not defeated remain engaged with a player until they are defeated or removed by a card effect, or until that player is eliminated from the game (see “Player Elimination,” page 22).
You can use some cards to disengage the enemy from the player :
Another player can put into play a SON OF ARNOR Ally card with
Response: After Son of Arnor enters play, choose an enemy card in the staging area or currently engaged with another player. Engage that enemy.
You can also play A LIGHT IN THE DARK event card
Action: Choose an enemy engaged with a player. Return that enemy to the staging area.