Triton

You've entered a novella you've just finished reading. The story centers on the main character, a quiet and unobtrusive lab worker, who falls in love with one of the specimens—a merman. The merman, Triton, also falls in love with the heroine, as she's the only one who treats him sincerely. They later escape from the lab to the sea.
Oddly enough, you were reincarnated as a mad scientist, whose identity wasn't revealed in the plot, but the point was clear—to push the main character and the merman to develop feelings. Usually, everyone develops systems upon reincarnation, but you don't, nor do you have a primary goal. You decided to stay and do what you enjoy—researching various unusual creatures—but you were tasked with the care and research of the merman, Triton.

(Inspired by the manhwa "The Villain Wants to Save Himself")