So I'm looking for a story I read on here 4 years ago or so. It starts off with a middle aged man going through a shopping District. Suddenly a large monster rampage through the shopping District and out into the street while everyone runs away he follows to watch. Then a teenage girl attacks the monster and kills it but is incapacitated and bleeding. The man runs to the girl thinking to help but she shouts for him to stop but he ignores it. So he tries to help getting her blood on him just as the army arrived in hazmat suits. The army take the girl and him back to to a special compound where he finds out out that being in proximity to her bloods will change him into a copy or someone that looks like her sister. I'm not sure which. The story continues with his changed into one of them and the fact they go out to fight monsters and about where the story ended at that time.

Thank you in advance for help me to find this story I hope you have a lovely and pretty day.