Μ-No.12-

μ-No.12-, commonly abbreviated as Mu-12 and known as both Kusanagi and the Sword of the Godslayer is a Boundary Interface Prime Field Device, Murakumo Unit, and the alter form of Noel Vermillion. Originally birthed in the 5th Hierarchical City of Ibukido, Mu was a depressed and murderous Prime Field that remained as a repressed part of Noel's soul. This part of her has disappeared with time, and her powers are used by Noel in times of need.

Mu first appeared as the final boss of BlazBlue: Continuum Shift, being unlocked once the true ending of story mode is completed.

Central Fiction
During the confrontation with Ragna, Mu was split from Noel’s soul, becoming her own separate entity, but maintaining Noel’s memories and personality. She too was absorbed into the Embryo, but a different part of it, a world separated from that the others were in. In this reality, she had murdered many people and friends. She found Makoto, unaware of the circumstances she was in, and battled her, knocking her down after her former friend attacked her.

Now inside the NOL branch of Kagutsuchi, she found Tsubaki, who turned her blade against the Godslayer, asking if it was fun when she killed so many people and took their futures from them. She turned into the Izayoi and the pair fought until phenomenon intervention took Izayoi away. Mu began to lament over her position, but resolved to find the cauldron, finding Kagura blocking her way – he made her lose control, and she depressed herself into no longer acting like Noel, but like the original Mu-12 – she killed the illusion of Kagura and the cauldron opened itself up.

Inside the cauldron, Mu discovered the Gate being protected by Es, the guardian. The two briefly battled before Es defeated her and sent her back to the world was previously in. There, she was found by the real Hazama, who managed to break her into a robotic trance and ordered her to kill the real Ragna. She complied but was broken from the trance by the latter, and she directed him out of the event into the one he came from. Mu attempted to follow, but she couldn’t, and asked Ragna to bring Noel back to a cauldron so she could fuse with her former self once more. Ragna did fulfil his promise by bringing Noel to a cauldron, but she was rejected, and had to wait for her other half to be able to accept her.

Much later, Mu was accepted by Noel when the latter used an Azure Grimoire as a cauldron. With this Mu finally reunited with the other half of her soul. Noel would later use Mu’s power to fully absorb Izanami and Saya into herself, becoming a complete whole of Saya.