Thanks for the clarification. That small misconception (I do want to believe it was unintended) did cost me the game. Looking over Defend's text and with your input, PredatorSmurf, it is now crystal clear to me:

Defend: Any adjacent friendly character may replace its defense value with this character’s unmodified defense value.

So the "may" is very important here...

Right. You don't have to do it unless you want to and when you do it lasts just for that one attack or action in the case of support.