The Hyakuya Orphanage Family refers to the children that Yūichirō Hyakuya and Mikaela Hyakuya lived with at the Hyakuya Orphanage in the Seraph of the End series. Within the manga, there are various unnamed orphans forming a part of the group. Within the anime, members of the family have names revealed.
The director of the orphanage had a policy that everyone there was family, so members share the same Hyakuya name after the orphanage, yet the children are not biologically related to one another.
They were neither formally adopted when they were captured by vampires after the apocalypse event and so are not adoptive siblings. However they maintained that they were family and continued to live together in the vampire city, Sanguinem, underneath Kyoto. There and beyond they would uphold that they were family.
As the apocalypse event is underway the orphans make their way back to the orphanage and are captured by vampires who are gathering the remaining humans to secure their blood as a food source. After living underground in livestock clothing for four years it is remembered how Yu joined the orphanage and met Mika first.
The family members join in an escape plan arranged by Mika, Yu and Akane however at the exit it is revealed as a trap set by a vampire noble, seventh progenitor Ferid Bathory. The orphans are all murdered by the supernaturally strong vampire aside from Yu.
Orphans including Akane are visualized by Asuramaru who presents the scene of them deceased to Mika. They were used to exploit Mika's deepest fear that he is too weak to protect his family and that he would be blamed for their deaths. Illusionary members of them have been created by the being imitating Yu who are woken up by him with the alert that they have to run away. One is happy to see Mika enter the room, then a girl in her futon sees him raise a knife. With the scenario clear she screams. They lie still on the floor of the room.
In Yu's memory that Demon Mika is accessing, orphans are seen deceased in the Sanguinem exit hall. They are also shown when Yu learns that he and Demon Mika share the same deepest wound in their hearts. They remain while the memory of young Yu engages with Mika, as Demon Mika tells Yu as he is presently that he remembers his name but not who he is.