The first time a sign of anything was going on between them was when Nasutei had to drive Seiji to Mount Dojo to save Shuu. Now, people always say Seiji was the first to check Nasutei out. Actually, I have to protest. It was actually Nasutei who checked Seiji out first. They were driving and talking and when Seiji leaned his head against the car window, Nasutei smiled all sweetly at him. That tells you first hand that Nasutei thought quite a bit of him. You could already tell she thought he was sweet and kind.
Later on, you can see that Seiji was very protective of her as he yells at her and tells her to stay behind for her own good. He tries covering it up by saying she would get in the way. We all know he just didn't want to get her hurt. He is constantly protecting her and telling her to stay behind for her own good. He acts cold, I believe, to cover up how he really feels about her. I also think he tries to protect her from himself. Maybe he is afraid with his cold personality that he would say something that would hurt her feelings or something. Seiji would never want anything to happen to her.
Seiji is also usually the one to agree with everything Nasutei says. In numerous episodes, we see that he agrees with all her plans. In the episode where they are looking for Touma, Nasutei says that two of the Troopers should go and look for Touma while the other two head back to town. Seiji was the first to say that was a good idea. Also, in a later episode, she said she would fix something healthy for dinner and Seiji was the first to say that it sounds great. He seems to be all for her and her ideas. Not only that, but Nasutei seems to agree with most things that Seiji says too. I think she admires him for his common sense.
Nasutei usually seems worried about Seiji too. In a few episodes, you can hear her call out his name in pure worry. When Arago dragged Seiji in by his neck in that one episode, "The Armor Must Be Destroyed," and then later Kayura showed up with Shin, Shuu, and Seiji tied to a pole, she was the one who called out Seiji's name. She also called out his name when he was battling Anubis (Cale) at Mount Dojo after he jumped on a cracked ledge. She also jumped off with him after he was falling. She would do anything to protect him.
Isn't it also pretty strange how he seems to show up just at the knick of time to save her? In "Splitting The Stone," he saved her right before she dies in the ice. He also saved her and Jun before they were run over by the train in that one episode.
There are numerous artbook images and episode images of the two of them together as well. I have a friend who went to Japan for a visit and said there were tons of artbooks with images of Nasutei and Seiji together. Is that proof or what?
I believe Nasutei and Seiji have a Silent Love. They seem to like each other, but do it in a way that only they know about. Its not very out in the open, but I definately think its there. I am very impressed with their relationship. It is even hinted in later episodes that they hook up. Many of you out there think that she and Ryo are a thing as well as others who seem to think her and Shuuten (Anubis) go together. ??? I for one, think that she and Seiji are an inseparable pair, and these are my reasons for believing so.