Zero didn't have many souls to work with and every single soul was a precious resource. Wii and Lily were as nervous as Zero but they tried not to let it show it in case it affected Zero mentally.
The doctor did his best to find the right balance between qi and mana. If qi was for the soul and mana for the body, Zero would have to be very careful about maturing the body and tying the soul into it before it starts accepting other scheduled souls.
The idea was fairly simple but it was harder than it sounded. Zero borrowed the concept of soul binding spells from necromancy. That was going to be the first step. He had to maintain that connection constantly between the future vessel and the trapped soul while supplying both qi and mana to stabilise both parts.
