nozaki kasuga
Nozaki Kasuga is the professor in charge of research for the ISDA's D-Project and a genius whose work in the past two years has heralded in a new era of dragon understanding. He is a quiet, well-mannered individual with a sad, almost haunted demeanor. Unlike the militant members of the ISDA, Nozaki doesn't wish to harm the dragons. Instead, he is aims for a peaceful future shared between dragons and humans alike. It may not come as a complete surprise then, that Nozaki Kasuga is actually one of the three original dragons involved in the ISDA 029 shuttle accident. He is Atrum, the injured dragon Kitajima Yuuri discovered in the aftermath of the crash. Disappearing after receiving Kitajima's touch, Atrum later reappears in human form under the assumed identity of Nozaki.
The three original dragons were initially sent to retrieve the eggs Thanatos left on Earth. After performing a Resonance with Kitajima however, Nozaki allied himself with humans and decided to protect the eggs so that they could be born on Earth instead.
Nozaki is a fairly troubled individual. After being gifted with a heart by Kitajima, he struggles to overcome the guilt he feels for being involved in the ISDA shuttle accident. He wishes to repent for his sins and forge a new path for humans and dragons to walk together. Unfortunately, as a probe dragon, Nozaki has a very limited life span and cannot hope to see such a future eventuate for himself. He chooses to sacrifices his life to give Toa and Jin the chance to communicate with Thanatos about the importance of human/dragon relationships.
with kitajima yuuri
Nozaki is defined by his relationship with Kitajima. Without her touch, he would never have existed beyond his dragon shell nor would he have come to experience human emotions. The incredible bond that was forged during their Resonance influences them both a great deal over the course of the series, even if Kitajima is unaware of its existence until half-way through the story. As we discover, even a Resonance as deep as Nozaki and Kitajima's is not without its complications.
Kitajima is a brilliant researcher with ambition but a severe lack of self-belief. She clings to materials and specimens instead of relying on her own theories and intuition. Nozaki acts as both mentor and advisor to Kitajima as she struggles to obtain another original dragon for experimental purposes. He repeatedly tells her not to rely on such things, that she doesn't need a live sample to solidify her research. Unfortunately, Kitajima is headstrong and unwilling to listen to Nozaki's advice. She feels inadequate when compared to him and desperately wishes to exceed his expectations of her.
What Kitajima doesn't understand is that right now she isn't looking for any original dragon, she is seeking Atrum — the dragon she subconsciously connected with after the shuttle accident. Album, Ravum, the Earth-born dragons — they are all substitutes for something Kitajima doesn't even consciously realise she is seeking. Nozaki knows he is her goal but feels he must keep her at bay to protect them both. And this is ultimately what he seeks to do — protect Kitajima (and her species) from harm.
Nozaki had a difficult time dealing with his involvement in the shuttle accident, even though he didn't actually do anything wrong (it was, after all, just an accident). During the ISDA flight memorial he shed the single tear Kitajima was unable to cry herself and was subsequently confused by its significance ("What is this? Why would I do that? I..."). As he reveals in the final episode, after completing his Resonance with Kitajima he had been plagued with concern. He felt that because his emotions were generated by the Resonance process, it meant that he lacked free will. But, as he concludes to Toa, this didn't really matter after all in the end - "The only thing that mattered was if those feelings were true for me."
The relationship between Kitajima and Nozaki reaches an important point after the former loses Toa to the Gillard. At the same time, Nozaki is aware that Thanatos has sent another emissary to finish the job Toa couldn't — a dragon sent to destroy the "tainted" Earth-born dragons. He tries to keep Kitajima out of danger but, unaware of the development within Thanatos, she defies his orders and returns to ISDA headquarters. Before the fourth original dragon reaches Earth, Nozaki decides to dispose of Ravum's corpse to let "her" rest. Kitajima becomes distraught when she sees Nozaki destroy this precious "sample". It is then that he reveals to her his true form, that of Atrum, and is given a savage beating from the dragon Ostrum for daring to protect Kitajima (and side with her species over his own).
Ostrum: Why are you allied with the humans?
Nozaki: Because that is my will.
For Kitajima, it is as though the pieces of a puzzle have finally come together and she goes against her logical thought process by choosing to stay by Nozaki, even after they return to the ISDA. He tells her that his mission was simply to collect and return the dragon eggs but after their meeting things became complicated, "You changed me". Their Resonance caused him to re-evaluate his position and decide for himself whether or not Earth should be abandoned. Nozaki chose to protect Kitajima, her world and all the new sensations she'd given him: effectively making him an enemy to his own species.
When the ISDA separates before the time skip, Kitajima and Nozaki decide to stay on Earth to distance themselves from the agency after Nozaki's secret was revealed. Before departing, the pair manage to spirit away the entire container of dragon eggs — opting to protect the unborn dragons rather than leave them in Sakaki's increasingly unstable hands.
After the time skip, the pair are shown to have taken up residence in the Akihabara district of Tokyo. They remain in hiding, protecting and observing the eggs while they work on developing a device that allows dragons to use a considerable portion of their power when under CSS fields. At one point, Kitajima asks Nozaki whether he thinks dragons and humans can truly co-exist — his response is quite telling.
Nozaki: I wish for it to happen. That's why I saved them (the eggs) from the ISDA. I can understand Sakaki's feelings about disposing of the eggs though. When I look at the Mars disaster and think about how much power may be dormant in me, I have my doubts.
As Nozaki struggles with his health, Kitajima stands by his side, assisting him as best she can — just as he helped her while she was still ignorant of who he truly was. During this time Nozaki is entirely accepting of Yonamine's reappearance in their lives. The three of them work together during the final arc to help liberate the captured dragons from Tartarus and protect the remaining eggs from Gio. Before going into battle with Ravum in episode twenty-one, Nozaki asks Yonamine to "take good care of her " with the pair seeming to come to a mutual understanding with regard to protecting the woman they both care about. When Gio attacks the dragon eggs, Yonamine and Kitajima stay by Nozaki's side, risking death for their cause.
Nozaki's ailing health (due to the stigmata on his back) becomes key in the final arc of the series. When presented with the opportunity to absorb the human-generated "Anti-Thanatos factor" from a dying Laura, Nozaki seizes his chance. By taking in this genetic time-bomb, he seals his destiny and begins to commit suicide. Though Kitajima begs Nozaki to stay on Earth, he refuses — deciding instead to take the Anti-Thanatos factor to Thanatos herself.
In space, Toa and Jin (with Gio) are desperate to plead their case for human-dragon coexistence to Thanatos but an army of Ostrums hampers their attempts. Nozaki appears beside them and offers to sacrifice himself in order to open a direct path to the mother dragon. When asked "Why?" Nozaki simply replies "I have someone I want to protect — would it be strange if I said that?". This echoes a quote by Jin's father heard earlier in the series ("I have something I want to protect"). As Nozaki sacrifices himself to save all that they have come to know together, Kitajima senses his demise and begins to cry. His final message to her comes in the form of a single word, "Yuuri".