Noya's interactions and relationships with other characters in Seraph of the End.
Mahiru is the one who prepared him as a demon weapon. She chatted to him about Guren. Mahiru eventually chose to die by stabbing herself with the sword and integrated with it into her heart. The sacrifice resulted in her dying in the process and caused Noya to fuse with Mahiru, turning them both into Guren's current demon Mahiru-no-Yo. However it was revealed that Noya is still alive with his consciousness still intact where he can be separated, evident when he erupted from Mahiru and Guren was able to call his name.
After being separated from Mahiru-no-Yo's body, he acts like a child towards the spirit of Mahiru, feeling superior to her as a demon. Being confident he is free now and cannot be merged with her anymore, he also expresses an intent to come over and eat her instead.
Guren is the human Noya makes a contract with in volume 4 of Guren Ichinose: Catastrophe at Sixteen. Though he mocks him, he also makes use of righting wrongs to tempt Guren with desire and power, and to become a demon.
At some point during the Dark Ages of Greece, Noya seems to know quite a lot about him, but appeared wary as a small Yu takes interest in the corpse of "Mikaela" after Noya brought it to attention. He tells Yu he could call it a fun toy when Yu believes it to be that, and Noya helps him see what is in the coffin before killing Yu via decapitation. Noya did this believing it would save himself, Ashera and Krul and thinking that it was not a problem as Sika Madu will create more Yu's.
In the present, he was surprised to see a fully grown Yu.
Noya, along with the spirit of Mahiru and Guren each used a sinful key to successfully restrain the first and rescue Shinoa from possession. Despite that action, Noya laughed at Shinoa referring to Mahiru as elder sister and advised that they kill her and be done.
Noya along with Krul and Ashera in the Dark Ages of Greece there is seen a corpse of "Mikaela" whom what all of them were meant to become but failed. It is unknown what Noya, Krul and Ashera were trying to do to it. However in the present, he is yet surprised to see a "Mikaela" alive.
In the Dark Ages of Greece, Noya along with Ashera and Krul and a young Yu were one of Sika's room where he keeps his "Mikaela", which was forbidden to enter. Despite that, Noya was the most willing to go behind Sika Madu's back to investigate this secret, and refers to him as 'Sigama'.
In the present, Noya joins Guren and Mahiru in fighting Shikama Doji where Noya was able to grasp of scythes flung towards them. Noya also had a hand in restraining him using a sinful key. Later when he saw a fully grown Yu and then Mika he wonders aloud if Sika Madu has managed to get the "human creation thing" to work.
In the Dark Ages of Greece he was along with with him, he and Ashera were just acquaintances at the time. However in the present, he seems to be interested in him after he found him in Yu's sword seeing it as where he ran off to. As Asuramaru he considers Noya a dangerous demon and a threat and was willingly to even give Yu all of his power. Noya confronts him calling him as 'Ashera' in Yu's body, at first he tries to remind him of himself he mentions that Ashera was the scariest one of all of them, and after being called by his name, he is delighted to have remembered him, then after being overpowered by him, Noya notes that he must be working with his human host Yu.
In the Dark Ages of Greece Noya was there along with her, and did not think much of Krul's concerns that they had been seen by Yu, being convinced that their secret was still safe. Though Krul disliked him for his actions, Noya attempted to alleviate her concerns. Krul is also concerned about his actions in the present since he is after her brother Ashera.
Noya, along with Mahiru both together make up the demon Mahiru-no-Yo. This being shares Noya's goal to possess humans and tempt them with desire.