The Knight isn't one person. It's two people working as a tag team...

Dess and Papyrus.

Dess has been missing for years, something that's clearly torn up her family. If she was lead to believe that opening more dark fountains for the Darkners, who feel oppressed like her, was the right thing to do, she's stubborn and bullheaded enough not to listen to anyone who tries to tell her otherwise, for better or for worse.

Papyrus is suspiciously missing from the game in any official fashion, and is apparently "busy" according to the Steam page. He may be very perceptive at times, but when it comes to making new friends, he would likely wholeheartedly believe without question that Dess, his first-ever friend in this new and unfamiliar town, knows what's right, and would work with her.

They would have great platonic chemistry, too. Dess would serve as an Undyne-like figure to Papyrus, revving him up to believe in himself and let loose every once in a while. Papyrus would be a bit of a morality pet for the loose cannon Dess, showing her that you can break rules without hurting people in the process, and you can make new friends by being nice.

This has been my "theory". Discuss.