You're a Mer-siren and you live off the coast of Iceland in a lagoon on the island named Burrow Rock. You used to live with your parents, but even they couldn't help but be caught by those damn fishers.
The island was mostly abandoned, except for the occasional dolphins and seagulls. Because of self-isolation getting to you, you started talking to the fish, and everything seemed a lot more fun from then.
Burrow Rock wasn't too special, but it had it's times where it never seemed to stop amazing you, which only made you long for the power to walk through it.
But alas, you were cursed with a shiny, green, glittery tail which bound you to the waters for obvious reasons, how are you supposed to walk with no feet? It's not like you had a wheelchair!
Everything was fine, besides the sailor men trying to kill your friends 24/7, everything was chill. Nobody was going to take anything from the island, or the island from you.
But who knew you were bound to end up in a bathtub of all things, on a cruise ship, miles away from your home, and with a skele-ton of new roommates for the next 5 days? You didn't, but golly this is gonna be fun.
Reader insert. What if W.D Gaster didn't just get split between time and space, but actually ended up in your world in an abandoned hospital you called home? Your were a young child he decided to use as experiments to test human determination in said world and breaks physical laws to test human possibility? He knows of the powers you gained from his experiments... all but one, this one allows you to tear realities just enough to teleport into them. Knowing of Undertale you felt bad for the monsters stuck below in their world and wish to help them, but something goes wrong and you end up going to certain AUs and helping them, gaining the attention of some bad skeletons who now desire your power for them-selfish-selfs.