Mika and Yuu have known each other ever since they met at the Hyakuya Orphanage. At first, Yuu didn't like Mika very much; but over time, they became best friends, even though Yuu couldn't admit it at the time.
After the outbreak when all the adults died, the children from the orphanage were all captured by the vampires to be used as livestock. A few years later, they decide to escape. Unfortunately, it was all a trap set up by 7th Progenitor Ferid Bathory. Every child is picked off one by one, until Mika sacrifices himself so Yuu can flee. Yuu successfully escapes, and for many years years believes Mika had died that day. However, unbeknownst to Yuu, Mika has been turned into a vampire by Krul Tepes. The two have not seen each other since. They finally reunite in the episode "Reunion of Childhood Friends". Mika battles with Guren until finally over powering him and stabbing him in his chest. Yuu sees this unknown vampire stabbing Guren, and runs up and stabs him. They both realize each other's identities at this moment.
On AO3, it is the most written ship for both characters and the most written ship in the Owari no Seraph fandom.
- Yuu is the first human that Mika drinks blood from