Never Bring a Knife to a Gun Fight
Atsumu Miya, best friends to the leader of the most well-known gang in the Tokyo prefecture (the Black Jackles), is sent on a mission to infiltrate the base of a rivaling gang in the same prefecture (the Serpents). His plans get overthrown when he is caught by the boss' most valued member, Kiyoomi Sakusa. Throughout their several interactions afterward, they're eventually closer than ever, which reveals more than just hatred towards each other.
(This story is based on Shakespeare's "Romeo and Juliet" piece, and a mix of two stories I've written (but deleted) like this.)