Bill is a siren and Dipper is a human both 18
1800s in the forbidden forest

"Man I hate being a proper lady when men can do whatever!" Dipper whined and kicked pebbles as she walked further into the forest walking past many trees till she came upon a pond and near a crystal cave.

"Go back~" a voice said making Dipper jump clutching her heart.

"Calm down Dipper nobody is here! Just going for a swim." She said then took off her puffy dress even her undergarments then dived into the water.

"Big mistake~" the voice said again making Dipper panic looking around hurriedly.

"Hello? Anyone there! I swear if there is a peeking tom out there I'll tell my Grunkles!" Dipper threatened then turned to see a badly beaten looken gnome "Aw!" She screamed.

"Your in its territory now." The gnome chocked out then a golden tail in the water took the creature underwater where Dipper saw air bubbles till it stopped making panic again and tried swimming away but the same golden tail pulled Dipper underwater as well making her gasp and struggle to breathe as she passed out.
Later
Dipper gasped awake breathing heavily taking in the cave she was in still in her birthday suit.

"Hello there I'm Bill whats yours My fair maiden~" This siren known as Bill said slithering from behind her making her hitch a breath and cover herself up "Humans and there constant need to cover what's been given." Bill shook his head smiling.

"What do you want?" Dipper managed to say to the siren making him chuckles and squeeze her with his tail making her immobile.

"You have captured my interest so I've decided to make you my mate." Bill said making the girl recoil as he takes her face and kisses her.