"The Creature of shadows which its in Two Halfs"
-Selena mentioning herself and Sarah
Selena and Sarah Are two Shadow Creatures In UnderWar.

BackStory(I know its crappy But i dont Care)

Gaster has screwed around with Void energy, what if when he was experimenting, he split a monster in two and created them. One escaped (Selena) And the other was still there, being tourtured by Gaster

And Selena feels all the pain of her other half as she tries to find out what Gaster is doing with his studies, she has scars and stuff from all the experiments on Sarah.

Currently Selena is trying to Search of her other half.