Gokudou Parasites

Mai Shintani, a young police officer, is suddenly assigned to the Organized Crime division due to a lack of staff. On her first night on the job, she encounters Shougo Mikuro, leader of the violent Tatsugami-gumi and the most dangerous yakuza in the city. However, he suddenly shrinks right in front of her eyes! Now small enough to fit in her pocket, Shougo takes refuge in Mai's apartment, and in return starts to help her crack down on rival gangs.

[Written by Denji