So, something was bugging me about Monster teen's design, something i couldn't quite place until playing undertale again.

Now, in undertale, Monster Kid mentions having a sister, but we never see them, so we think. Monster kid has a pretty simple, but clear-cut design, especially for his foreword facing sprite.

His overworld sprite: https://imgur.com/B1X71ny

His Battle Sprite: https://imgur.com/wTFLG8o

Even his Tarot card: https://imgur.com/LpqtW41

Each time, his spikes always face to the right side of his body, and his "black eye" is on the right side as well.

But in Deltarune, the monster teen we see has a quite different design, not just older, but the spikes are completely on the other side of his body

Monster Teen's sprite: https://imgur.com/VvqzHr9

But.. we do have an example of this. In Undertale, in waterfall, if you're lucky enough, you'll meet Gonerkid, a black and white version of Monster kid, who seems like just a mirrored sprite at first.

Goner kid and Monster kid's sprites for comparison: https://imgur.com/sMA7XlI

My thought... is that the monster teen we meet in Deltarune, is actually the Sister that monster kid mentions in Undertale, and that in this universe, the original monster kid is a "Goner" of the Deltarune universe (at least so far, 5 more chapters for him to appear)