Yami Kyun!

Overview

By day, Miya Akatsuki is just a slightly grumpy and unsocial girl studying in St. Margaret's Girls' College. But by night she's secretly a Final Executor – an ultimate debt collector who can make even the shadiest debtors click the "accept" button on the payment warrant. The debt collection agency she works for, however, is a slightly different than normal ones. Haruki Kobayashi, a boy working in the electronics store of his family, becomes affiliated with Miya after accidentally witnessing her at work. She and her two colleagues end up moving to Haruki's home as part-timers to keep an eye on him. Gradually Haruki starts to learn about the painful loss that made Miya want to obtain power.

Source: AniList