Board Thread:Questions and Answers/@comment-30782973-20170522163127/@comment-27701762-20170522192503

As part of the game? Nothing. Asriel's coding almost certainly does not have a "Spare" setting, so his name probably wouldn't even turn yellow, and even if it did, it still probably wouldn't result in anything. That's your best case scenario. Worst case: it bugs out the game.

Per the rules of the world? Still nothing. Since the entire point of "sparing" is that you and the monster don't want to fight each other anymore. But Asriel's entire battle is about him wanting to fight you to take complete control of the timeline. So no matter how many times you spared him, he wouldn't change.