Noya Hienma lived as the past as a human and a vampire where he was acquainted with Ashera Tepes and Krul Tepes then familiar with the past Yūichirō Hyakuya and Sika Madu. In recent times Noya as a demon was initially bound to and wielded as a cursed gear sword by Mahiru Hīragi then Guren Ichinose who later tries to befriend him. Noya would be absorbed and fused into Mahiru's body where they became the fused being of Mahiru-no-Yo (a different entity with its own relationships section). With Mahiru-no-Yo divided Noya would be released by Guren. Noya Hienma as he is interacts with various other characters throughout the Seraph of the End series, which affects himself and the story. This article details Noya's relationships with other characters.
He was prepared as a demon weapon by this member of the Hīragi family and they spoke together about Guren. Mahiru eventually chose to die by stabbing herself through the heart with the sword and integrated with Noya who was bound to the weapon. 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. At being separated Noya retained his own consciousness and was aware it had been some time and he was able to recognize and engage in conversation with Guren.
Regarding the spirit of Mahiru he acts like a child towards her and displays superiority over Mahiru as a demon. Being confident he is free he also expresses an intent to come over and eat her instead. Whilst he co-operates with Mahiru during conflicts against others Noya shows fear at the uncertainty of Mahiru's plans. Noya is struck with an injection from Mahiru which renders him unconscious.
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. Seeing him again some time later this behaviour resumes at once as Noya entertains Guren's desires to get women, destroy everything he hates, then make him the most powerful demon who ever lived. Fighting beside him, Noya assists Guren in battling against Yu and Mika. Guren later discards Noya upon being struck with an injection from Mahiru, he then falls unconscious.
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 "Michaela" after Noya brought it to attention. He tells Yu he could call it a fun toy. When Yu believes it to be that, Noya helps him see what is in the resting place before killing Yu via decapitation. Noya did this believing it would save himself, Ashera and Krul and thought that it was not a problem as Sika Madu will create more Yu's. Noya visits Yu when he tries to wake up Michaela, believing he will wake up if he is there. Noya calls him an idiot as he won't wake up because he is dead and finding him annoying he kills him. Ashera comes barging on Noya upon killing another Yu, but Noya doesn't care as ton of them will be created over and over. Noya explains that this "Yu" was simply loud and annoying. Noya makes another Yu by using one of his eye as a cell, growing into another Yu, however he immediately kills him since he was very loud, annoying Noya which makes Ashera sick. Another one is created, and Noya warns him if he became loud he will kill him.
In the present, he was surprised to see a fully grown Yu. With Yu's mouth bound Noya instructs him not to mumble then that it is not him he is after. Along with Guren and Mahiru, he fights Yu and intends to take his head.
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 her elder sister and advised Guren that they kill her and be done.
Noya along with Krul and Ashera in the Dark Ages of Greece enter a forbidden room to view a corpse of "Michaela." Noya states that was what all of them were meant to become but failed. It is unknown what Noya, Krul and Ashera were trying to do to it although Noya had something in mind. It is later revealed that the Michaela is Sika Madu's son.
In the Dark Ages of Greece, Noya along with Ashera and Krul and a young Yu were in one of Sika Madu's rooms where he keeps his "Michaela", which was forbidden to enter. Despite that, Noya was the most willing to go behind Sika Madu's back to investigate this secret. Noya referred to him as 'Sigama'. Even though Sika Madu already knew about this, he didn't care about them going into his forbidden room, including Noya killing the Yu's that were cloned.
In the present, Noya joins Guren and Mahiru in fighting Shikama Doji where Noya was able to grasp a large scythes flung towards them. Noya also had a hand in restraining the first using a sinful key. Later when he saw a fully grown Yu and then Mika as well Noya wonders aloud if Sika Madu has managed to get the "human creation thing" to work.
In the Dark Ages of Greece Noya was familiar with Ashera and investigated the corpse of Michaela together. Ashera disliked his behaviour when he was going against Sika Madu's orders and killing the Yu's that were being created simply because they were annoyance to him. In the present he seems to be interested in Ashera after he found him in Yu's sword and considered it as where he ran off to.
Noya addresses him as 'Ashera' whilst he is in Yu's body and 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, Noya is delighted to have been remembered by him. After being overpowered by Ashera with abilities he is unfamiliar with, Noya notes that he must be working with his human host Yu.
In the Dark Ages of Greece Noya was there along with Krul and he did not think much of her concerns that they had been seen by Yu. He responded to Krul's apprehension by saying that their secret was still safe. Though Krul disliked his actions, Noya attempted to alleviate her concerns.
In the present Noya is pleasantly surprised to see a "Michaela" alive and regards him as Sika Madu's first successful attempt to create one. Along with Guren he pursues Mika along with Yu.
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. Noya was not active at all since Mahiru was suppressing him. Noya extremely hated that when he was separated from her, as he wanted Guren to use him instead. Separated, Mahiru-no-Yo is still invoked as a weapon where it is used to assail foes.