Kekkai Sensen - Back 2 Back

Kekkai Sensen centers around the crime fighting organization "Libra" and their battles in Jerusalem's Lot, formerly known as New York City. Jerusalem's Lot was created in a single night when a portal to the "Beyond" opened, and now it is a paranormal melting pot of monsters, magic, and the everyday mundane city life. It's up to Libra to prevent the horrors of this city from spreading to the outside world.

When mild-mannered Leonard Watch arrives in the city, he's inducted into Libra and things spiral out of control from there.

(Source: Wikipedia)