Smol Undertale OC. Long bio:
Allein (pronounced al-line) means alone in German.

Allein's mother was killed by a human when she was only 6 years old. Her sister, Undyne, was badly injured and lost one of her eyes. The two lived with their father until Allein was 14 years old and Undyne was 18 years old, when their father died. Undyne worked hard to support Allein but struggled due to Allein's constant illness and generally weak state. Papyrus became friends with Allien but Sans was hesitant for some reason. He tried to persuade her to take off her necklace many times but Allein's mother gave her that necklace and she never takes it off. One day Sans tried to forcefully rip the necklace off of Allien but Undyne stopped him. Undyne never let Sans come in her house or near Allein again and she harbors resentful feelings towards him because of the whole incident. Undyne and Allein remained friends with Papyrus and Sans's name even being uttered is still considered a slur by Undyne.

(So, yes this is my OC. She is Undyne's younger sister. If you read her bio, I would just like to say that Sans was not necessarily in the wrong, but Undyne interpreted him trying to rip Allein's necklace off as him attacking her. About the necklace...Sans simply knows something that we do not. And if you're going to ask how the dad died, I'm not going to reveal that right now. I will write more of her backstory that will reveal all of these mysteries.)