Mahiru's lover, as she intentionally chose to die by Guren's demon by stabbing herself with his previous weapon, and integrated with it in her heart; sacrificing herself and became the demon in his sword.
Mahiru's sister, when she hears Guren invoke his demon name, she confronts Guren and asks him if he is in control of Mahiru-no-Yo or is she controlling him. She is still angered at the fact that her sister is within Guren's sword and is convinced that he killed her, despite it being the latter.
Guren's previous demon given to him by Mahiru in the light novels in volume 4 of Guren Ichinose: Catastrophe at Sixteen. Mahiru used it to stab herself with it and integrated with it in her heart as Noya's curse enveloped her, Mahiru was able to use Noya by sealing/fusing herself with it transforming it to Mahiru-no-Yo. However Noya managed to erupt from Mahiru-no-Yo's demon body, meaning he didn't completely disappear after he was merged with Mahiru and Guren was able to call his name as well.
Mahiru first meets Byakkomaru when Guren Squad was heading to Osaka. He manifests in front of Mahiru-no-Yo and confronts her, he is cautious of her since he can tell she is not a demon, he threatens her if she tries to harm Shinya which Mahiru silences him and ignores the warning.
A seventh progenitor vampire who appeared in Seraph of the End: Guren Ichinose: Catastrophe at Sixteen and has been working with Mahiru for quite sometime. However upon Mahiru becomes Guren's Cursed Gear, she knew Ferid would approach him and use him. As he reappears in Seraph of the End: Guren Ichinose: Resurrection at Nineteen. Thus Mahiru-no-Yo ordered Guren to kill him and she's not above to possess him in doing so. She see's him as someone who gets in Guren's way as she is the only one who Guren should listen to. Upon learning what happened to Mahiru as she became a demon, Ferid is amused by this fact, and he leaves them alone.