Princess

  Brought by one of the teachers. Never truly belonged. Most of the Darkners were creeped out and avoided her. Seeing how beloved the current rulers were, drew conclusion she will be too, if she takes their place. And it worked. Despite taking the throne by force (unbeknownst to everyone except for the rulers themselves), she does care for her people and takes her duties seriously.

  Lighters, Darkners - Princess is neutral, benevolent even, towards everyone upon first meeting, and for as long as they stay in line. Which is not that hard, considering that only includes not actively posing a threat to her reign. She got along swimmingly with Avery because of that and also because she was the first person not to freak out over her appearance. Although Princess suspects it's because they look similar, not noticing her new friend's unfazed by everyone else whose looks couldn't be farther from humanoid.

  Princess is perfectly content with her small world and couldn't care less about opening more Dark Fountains; the Knight can manage without her. What she does care about is that a bunch of randos came to close her Fountain and take away everything from her. Reasons and possible consequences be damned, she will not let this happen. So she lies to her people that the Fun Gang came to destroy their world to get the Darkners to stop them.

  Good at pulling metaphorical strings, and for when that fails, she has literal ones. In the Light World, Princess's a marionette whose strings withered away, which somehow translated into her being able to control other Darkners, which extends to everyone she's ever touched. Very few are aware of this ability, and the sole reason they are at all is that they've got to experience it firsthand. These unfortunate souls are kept quiet by the threat of being put through this again. 

  Albeit harmless physically, it's quite trauma-inducing. Not only does the victim lose control of their limbs, they're rendered unable to speak and, more often than not, see (whilst Princess is speaking and/or watching through them). It helps little that everything goes back to normal once the control is broken. This ability is limited to one Darkner at a time. Princess also can't hear what her "puppet" hears, which resulted in her learning to read lips, just in case it might come in handy. Unfortunately, it does on True Pacifist.

  No Mercy is the only route where Princess'd be the final boss. She fights the party more so for the safety of her people, appalled that her words might hold water. Keeping her title is still a part of the reason, but no longer a top priority.

Trivia

 Despite frequently reminding myself the Darkners probably shouldn't look like humans since they were made by monsters, I failed to remember that while designing Princess. At least she lacks a human nose. Or any nose...

 Before my botched attempt at giving her depth, she was an irredeemable villain who enjoyed what she was doing. 

 Just in case there's Princess in one of the unreleased chapters, I've got a spare name for mine - Mary Annette.