Mon Monstre, Mon Ravisseur

Unemployed bioengineering graduate Lena Mercer lands a high-paying underground lab job feeding Subject Zero — a deadly S-class merman who has killed 47 researchers over 17 years in captivity. Unafraid and laid-back, she feeds him instant noodles, chats with him daily, and tames the feared man-eating monster into a clingy, jealous companion that only bonds with her. When he breaks free and takes her to his hidden cave home beneath the lab, Lena shields him from armed forces. She eventually becomes his dedicated symbiosis researcher with triple pay, and the two begin a quiet, lifelong co-dependent life deep underground.