Alphys, Ex Captain Royal Guard

She was perfect, the captain of the guard, the role model for the cause, Toriel's pride.

All of this crumbled the moment Sans challenged her to a duel for her rank and she lost miserably. The skeleton wiped the floor with her and turned her into a disappointment in the eyes of the queen: "If only you were better, if only you had what it takes, but no, you are incapable of that".

From being confident, proud, and haughty, Alphys transformed into someone with severe self-esteem issues, perfectionism, and a complex about always being second best to Sans.

Toriel is the closest thing to a mother she ever had, and being the light of her eyes from a young age was her main motivation. Losing that broke something inside her, and this transformed into a resentment against who "stole it from her" that burns every time it's brought up.

The queen, in particular, reinforces this whenever Alphys is in her presence, forcing her to wear her former captain's uniform but without the rank, to "prove that if she tried a little harder, she could be better".

What use is this to the queen? In her corrupted perception of things, it gives her a "card up her sleeve" in case something happens to the captain of the guard. Furthermore, it provides motivation to avoid mistakes, given that someone is willing to do anything to take his position.

And by everything, we mean absolutely everything. Alphys tries to sabotage Sans every chance she gets, whether it's by trying to make him fail missions or by upgrading her weaponry to the latest available technology.

She manages to do the latter by manipulating the Royal Mechanic to give her the latest creations before the rest of the guard.

Sans knows Alphys has a vendetta against him, yes, but his abilities are enough to get the job done. Or at least that's what he thinks, because Papyrus is keeping a watchful eye out in the shadows, preventing several of these sabotage attempts, and since a large part of his work is in Snowdin, the dino can't get there because of the cold.

🔺With Shaky on the run, word quickly spread that the seventh human had fallen into the Underground. While Papyrus tried to stop Sans, the kid arrived to Waterfall where they were unpleasantly surprised to find that the skeleton wouldn't be the only royal guard they'd have to escape.