Affected character must be declared as a defender during the "Declare defenders" step. He does not become a defender immediately after this event resolves.
This effect does not force players to make any choices or trigger any abilities in order to create a certain game state in which the affected character would be able to defend. Rather, when the time comes to declare defenders, if that character can be chosen to defend (e.g. is standing, has a corresponding challenge icon), he must be choosen.