#whoof.

This all started with the Miharu arc, once again. After Natsume asks Matoba the famous question, (have you ever thought about quitting exorcism?) and hearing Matoba so confidently state he's never considered it, he thinks

「名取さんは堂だろう。いつもゆらいでいる優しいあのひとは」

"What about Natori-san? That kind person who is always wavering."

The verb being translated as “wavering” is 揺らぐ. This word refers to a swaying or rocking motion, something going back and forth, or shaking/trembling. The reason this line stuck out to me is that it made me wonder "Why is Natori wavering? What does it mean to be wavering?" Thankfully, my questions were answered when I went back through the past exorcist chapters.

The concept of Natori feeling unsteady or his world feeling unsteady comes up in special chapter 15. When Matoba tells Natori about the history of the Natori clan, how even though they should have been especially able to deal with kami, they still fell, so "you never really know what will happen," Natori responds with "The Matoba clan might not always be on top either," and Matoba laughs and says "Who knows… I won’t let it fall."

This is probably when Natori stops seeing Matoba as merely the spoiled prince of a wealthy family, one who has everything Natori wanted growing up, and realizes what kind of burden he has on his shoulders. Indeed, after this exchange, Natori thinks "I don’t have anything like that resting on my shoulders. Maybe that’s why this unsteady/shaking/bubbling world seems to waver." (This is not how any English speaker would say this, but I want to convey the picture that’s being painted. He uses the onomatopoeia ぐらぐら which denotes boiling/wobbling/unsteadiness to describe the world). Again this verb 揺らぐ comes up when Natori says the "world seems to waver." Natori feels like he doesn’t have a defined place in the world the way Matoba does. He doesn’t know what kind of person he should or will be, unlike Matoba, who probably grew up seeing exactly what he would become in the previous clan head, down to the long hair and eyepatch. For Matoba, the world is steady, unmoving, and almost preordained. For Natori, everything is uncertain, including his own identity. 

I’m going to take a brief detour from the “wavering” idea to touch on something Yorishima said to Natori in chapter 80. Seeing that Natsume is emotionally affected by the story of the enchanted scarecrow, Yorishima remarks that Natsume’s heart has been pulled in by ayakashi, or in other words, he’s become emotionally involved with them. He tells Natori "A savage exorcist like you shouldn’t drag a (this kind of) kid around," and then "Natori, lately you’ve come to resemble the Matoba brat." Keep this idea in your back pocket. It’s a surprise tool that will help us later. 

Then, as I said before, in the Miharu arc,  after Natsume hears Matoba say that he’s never considered quitting exorcism, he wonders how Natori, who "is always wavering," would answer. And of course, later in that arc Natori himself asks Matoba the same question, though this time Matoba doesn’t answer. Although the focus is usually on Matoba’s answers to this question, I’ve also thought it significant that Natori asks Matoba this. He seems to initially want to ask it after Matoba ponders about the previous Matoba head who failed to protect the Miharu family, but gets interrupted by the spell on the room.

When Matoba starts going on one of his "you should make use of useful things" monologues, he asks it for real. It’s still not entirely clear to me what makes Natori want to ask him if he’s ever thought about quitting, though all I can really think of is that hearing Matoba talk like this reminds Natori of how tiring and cutthroat this life can be. Natori, the one who wavers, has probably thought a few times about giving up, and hearing Matoba talk this way makes him wonder, "Haven’t you ever thought about quitting?" Just like how Natsume asked Matoba after Matoba told him how many people are out to usurp the Matoba clan. 

Finally we come to the Younger Cousin arc. When Mitsuru tells Natsume about how she met Natori, she says that she didn’t care whether she was going to be exorcised or not, she just wanted to know (and say) Natori’s name, so when he saw her, she told him her name (which is Mitsuru, but written differently than either of the names that would have been given to Natori’s cousin), and she saw herself reflected in his eyes, "wavering" (揺れて). So Natori’s heart was shaken, to put it another way, when it seemed for a second that he could have what he had always wanted. For a moment, he hesitated, and this was the weakness that the ayakashi latched onto. She read his mind and created a false reality where she was the cousin he wished he’d had.

Natsume believed she was not malicious, and went to hear her out before Natori could exorcise her for real, but Nyanko-Sensei warns him "Those who can’t heartlessly dedicate themselves are unfit to be exorcists. If you support ayakashi, Natori is just gonna waver." Here, he uses the verb ぐらつく which is related to the onomatopoeia from special chapter 15. Nyanko-Sensei is telling Natsume that if he tries to stand up for ayakashi, Natori will just become more conflicted and eventually be unable to "heartlessly dedicate" himself to exorcism. 

When Natori and Mitsuru finally face off, Mitsuru tells Natori that she just wanted to help him. Natori tells her

「親切ごかしはもういい。どちらが化物だかわからなくなりそうだ。」

which is something like "Enough playing nice/Stop pretending to be nice. I might not be able to tell which of us is the monster." Mitsuru then makes the final offer for Natori to stay with her in the illusion and be happy together, but Natori, now firm in his resolve, says that he was weak for wanting a cousin who could see the same things as him, that he doesn’t need that anymore, and that Mitsuru is an ayakashi, which is what he’s chosen to spend his life fighting. This is where Natori makes the decision he faltered on when he first saw Mitsuru. He throws away what might have been, knowing that it never could be, and decides to stay firm on his path as a heartless exorcist.

At the conclusion of this arc, Natsume apologizes to Natori for trying to stop him from exorcising Mitsuru. Natori smiles and says "Ah, you really couldn’t be an exorcist," probably realizing now that he’s set himself on this path, that Natsume could never be "heartless" enough to follow the same path. Natsume seems to pick up on this difference between them, because he starts to ask about Natori, who tells him "Don’t worry. It’s not something I’m burdened with. It’s something I chose. I’m not bound by anything." By the way, he uses the same "burden" word here that he uses when he tells Matoba in the Miharu arc that he doesn’t know the weight of inheriting a burden. 

TL;DR:

So, after all this, what I think happened is this: Natori is "wavering" from the beginning, unsure of what he should do or who he should be. Even as an exorcist, he has doubts and hesitation, which eventually causes him to slip up and be taken advantage of by Mitsuru in the Younger Cousin arc. Having been confronted with his "weakness," he decides finally to "heartlessly dedicate" himself to the path of exorcism. In this way he has become more like Matoba, which Yorishima seems to have had noticed the beginnings of much earlier. A savage, heartless exorcist: something Natsume could never become. After I originally came to this conclusion, I wasn’t sure if I was reaching, but then I read Midorikawa’s afterword for the Younger Cousin arc, which I translate thusly:

"...I’m glad I could draw the world that Natori-san, for whom the boundary between ayakashi and humans wavers more than even Natsume, sees even a little, as well as how ayakashi see it. Natsume has really only had limited experiences in his small world, and in contrast, Natori-san has a greater deal of freedom, but feels trapped. If I was able to draw the path that Natori-san came in on, and the path he’s going on, I’ll be happy."