Jan is a predator. A reader-insert character in this universe is almost certainly a human, a fledgling vampire, or a prisoner. The tension comes from the lack of power. Writers explore a scenario where you cannot fight back, only survive.
This is the angst version. Perhaps the reader is a medic or a researcher. You can't fight Jan, but you stitch him up after his raids. You see the machinery of his body beneath the bravado. This leads to uncomfortable intimacy. hellsing jan valentine x reader
Jan Valentine was a villain, a foul-mouthed mercenary, and a dead man walking. But for one night, he was the center of your world, and he made sure you’d never forget the name Valentine. Jan is a predator