Back when Emil and Halua were still young children, they were experimented on in order to make the ultimate weapon. The mansion was used as a front, while there were secret underground labs under the building. After excessive experimenting, Halua was transformed into Number 6, a skeletal monster, while Emil gained petrification powers and was dubbed Number 7. Number 6 went mad, however, and had to be chained and bolted to a wall to stop it destroying the mansion.
Emil and Nier, searching for a way to cure Kainé's petrification, find Number 6 in the underground labs. They fight the monster, and Emil absorbs it, gaining its powers but also its skeletal appearance.