Prequel to Sense of Loyalty which shows the background of Omi and Rou along with the context of the world they live in.
http://www.mangago.me/read-manga/sense_of_loyalty/
Desperate for the money to pay his brother's hospital bills, a young man accepts an old friend's offer: to be trained as a 'pet' to be auctioned to the highest bidder.
Oath Of Allegiance