If Guy A is defending with one character, and Guy B kneels his Aggo's Bow to kill it, is it unapposed? He isn't there anymore - but he was declared as a defender. I'm Confused

You determine whether a challenge is unopposed at the end of the challenge - if the defending player counts 0 strength, then the challenge is unopposed. By far the most common way for this to happen is for the defender to have no participating characters.

So yes, the challenge would be considered unopposed if all the defenders got killed.