easier to feel nothing at all
The façade Seishirou wears for most of Tokyo Babylon portrays him as a kind man who's always willing to help out his beloved Subaru-kun. The person behind that mask, however, claims to feel nothing at all for other human beings and gives them no more concern than he would an object.
Seishirou: I have never felt anything in particular. For the longest time now, I haven't been able to distinguish between "people" and "things." For instance... to break your arm like this... or to break a glass cup? Where is the difference? Corpses... wreckage... they are one and the same to me. I suppose in that sense I'm well-suited to be a Sakurazukamori.
But is that really true? How much of a mask does Seishirou really wear?
As I mentioned on the eye page, Seishirou's mask is only as good as he is. As someone who claims to be without emotion, it follows that he can only fake it to a certain extent. It's entirely possible for him to have learned enough emotion via mimickry of those around him to get by, but when involved in a bet where the stakes are his very emotions, mimickry doesn't cut it. And I think there are a couple times during the series in which his true feelings do slip through.
Case in point: the "Call" chapters. Subaru has a case where three girls are making prank calls and casting spells, believing themselves to be "special," reincarnations of warriors, fated to battle in 1999. (It's a lot of foreshadowing for X; I'm not sure whether or not it's intentional.) The girls are amateurs and are hurt by their own spells; Subaru tries to help them, but is exhausted by the effort it takes to combat their spells and help them at the same time. When he passes out, Seishirou puts Hokuto to sleep and intervenes, far less gentle than Subaru with his spellwork.
But it's what he says to their claims of being "special" and how they disdain the people around them that sticks out to me.
Seishirou: The most admirable people in this world are those with their feet on the ground... who work hard every day. Those people you call "normal." They wake up every morning and go to school and to work. They laugh, they cry, they suffer and they worry. And they try very hard to face reality and just go on living. If you choose to ridicule these "normal" people... how do you expect to live with them?
The most important question is why Seishirou is saying these things. Subaru and Hokuto are unconscious; he's not putting on a show for them. He doesn't have to hide anything. So why is he saying it? Is this something that he really believes? These words exist in the series for a reason, but Seishirou is the one saying them.
Personally, I think that yes, this could be very close to what Seishirou believes. I don't mean it in the sense that he respects people; quite the opposite, in fact. I think that to him, normal people are something for him to observe, study, and mimick, so that he can better deceive others. So that he can blend in. Seishirou is not a "normal person." So, to him, normal people are admirable in that they do all these things that he cannot.
And, as Tokyo Babylon shows, Seishirou is very good at what he does. Subaru never picks up on it; the only one who does see through his act during the bet is Hokuto. She engages in constant banter with him, and while she can tell when he's deflecting her questions or not being entirely honest, she can't see the whole truth. In the hospital after Seishirou loses his eye, Hokuto observes that he hides behind his glasses and can no longer do so without them. He can't hide that his show of compassion and kindness isn't who he really is.
Hokuto: Subaru, I want you to think carefully. Who went through the biggest change over the course of this incident?
Subaru: That would be... Seishirou-san.
Hokuto: No. He hasn't changed. The only thing that I would concede about him... is that he stopped pretending.
On an intellectual level, Seishirou probably understood other peoples' emotions. During the bet, he acts as if he understands and empathizes with people, but once he reveals his true nature to Subaru, it's shown for the farce it is. For a farce, though, it is one built on a great deal of imagined empathy. I think Seishirou is capable of caring about other people and seeing them as more than just objects, as evidenced by the fact he can go through the motions as well as he did. Even so, whatever he considered emotions were likely not what most people would. He might have considered it a kindness to leave his eye to Subaru, but it was at the price of his death, a price Subaru never would have paid had he known what Seishirou was doing. Seishirou probably could have explained his actions on a logical level, but emotionally? Probably not.
And yet, for all of his supposed indifference, it's unquestionable that Seishirou ended up caring about Subaru. It's not caring in the way any so-called "normal person" would think about it, no, but he cares insofar that things end up mattering to him. It mattered to him that Subaru lost his eye, and he consciously desired to change that, which Fuuma saw and carried out for him. (Obviously, there's the issue of love, but that's an extensive subject, so please refer to this page.)
Seishirou isn't alone in having trouble understanding emotions; so does Subaru. Subaru is the polar opposite of Seishirou, offering his help and compassion to anyone who needs it, but he doesn't understand his own feelings. It's because of that that it takes Subaru so long to realize how he feels about Seishirou, but it's not restricted to that. There are several instances throughout Tokyo Babylon in which Subaru puts the needs of others above his own — as I mentioned above, in the "Call" chapters Subaru persists in trying to help the girls making prank calls despite his weakened condition.
Hokuto puts it best in the "Rebirth" chapter:
Hokuto: He's always had this bad habit of making other people's emotions his own. When they're sad. Subaru's sad. When they suffer, Subaru suffers, too. He's not trying to be a saint. That's just how he is. But because of his job as an onmyouji... He's seen so many people's bad sides, and experienced so many unpleasant things... and he's never been able to get used to it. I'm sure that Subaru remembers all the emotions of every person he's ever dealt with. Even if they themselves forget, Subaru never will. He carries them with him. He can't let them go.
Take just a moment and imagine living like that. You'd be miserable. Which explains, of course, why Subaru himself is so miserable. He acts like a kind, caring child, but to take on the emotions of all the people you help and keep them — especially considering the kinds of clients Subaru has... well, let's just say the kid was screwed up long before Seishirou came around. (It also explains, quite neatly, why Subaru closed himself off so much after Seishirou betrayed him, and Hokuto foreshadows the ending in this scene, saying that if Subaru were to fall and love and be betrayed, he would break.)
Within the series, we hardly ever see him do anything for himself — we always see him serving his clients' wishes, first and foremost. Since the message of the series is that an individual cannot know the pain of another, Subaru suffers, time and time again, to try and help people with what they want. The one exception is in the "Crime" chapter, where he meets a mother who wants to get revenge on the man who killed her daughter. Subaru calls the spirit of the daughter back, thinking she would not want to see her mother do such a thing, but finds instead she's begging for revenge, though her mother cannot hear her. Subaru lies and tells the mother what he thought she would have said instead in an attempt to spare her from pain. He goes to Seishirou, who (ironically) consoles him.
Subaru: Maybe I was wrong about everything. Maybe I decided not to tell her the truth... selfishly. When only she can decide what makes her happy. That's right. I denied her the truth for my own sake.
Seishirou: You're right that only that woman can know what will make her happy. So by that logic... only she can decide whether your lying was the right thing for her or not.
Seishirou: Even if you were wrong to do that... could you blame anyone for making an unwitting mistake?
Seishirou: Then... you should forgive yourself.
Immediately following this scene, Seishirou reassures him and puts Subaru to bed. Subaru thanks him. Afterwards, Seishirou leaves the room, and, seeing a mirror, breaks it.
Seishirou: At this rate, I'm definitely going to win our bet... Subaru-kun.
Seeing Subaru in pain doesn't do anything for him. He comforts him, but it's only part of the bet. He lashes out at his reflection, I think, to signify that he cares nothing for the breaking of glass as he doesn't care for Subaru's troubled emotions. In the greater story of Tokyo Babylon, this story in particular is one of the few times Subaru is selfish. He places his feelings over that of the person he's trying to save. But he always cares more about doing the right thing than what he wants for himself, and that's why he could never have won the bet.
And after he loses, he breaks.