i'll make a bet with you
Let's talk about the bet.
Seishirou: If you and I should ever meet again... let us live together for one year. My heart is the direct inverse of yours. You are kind, and pure, and honest. And I'm sure that is how you will continue to grow up. Your heart will remain pure. So, if we should ever meet again... I will try my hardest to learn to love you. Just for one year. And after that year... if I can consider you "special"... you will have won and I will not kill you. But if I decide... that I cannot consider you special to me... if I cannot distinguish between you and that corpse... then... I will kill you. And so today... I will let you go.
Well, isn't that just the most biased bet you've ever heard. Like, okay, don't get me wrong. I love the bet as a plot device. I love the reveal, and I love the ending of Tokyo Babylon; it's why it's my favorite CLAMP series. But jesus christ, the bet itself is so ridiculously biased towards Seishirou that you have to wonder if Subaru ever had a chance in the first place. And then you throw in the fact that oh wait, Seishirou erased Subaru's memory, and... yeah.
Hey, Seishirou, you weren't worried about losing, were you?
Anyway. I've talked about this elsewhere, but I do think that at least some of Seishirou's motivation in making the bet in the first place was to find someone to love, so that he would have a successor. I don't think he planned for it to be, you know, the next Sumeragi clan head, but he still went through with it anyway. I do also think that he was genuinely curious if he could end up caring about Subaru, and that he put forth a real effort in trying to care about him. However, the bet was so ridiculously skewed in Seishirou's favor that Subaru had no chance of winning.
But did Subaru even lose in the first place?
After all, the conditions of the bet were that if Seishirou did not come to care for Subaru, he would kill him. And while Seishirou does break Subaru's arm and captures him inside his illusion, he doesn't fight back against Subaru's grandmother when she breaks his spell. He simply withdraws and leaves Subaru where he is, broken and empty. With the star marks on Subaru's hands, he could come for Subaru at any time and finish the job, but he doesn't. It's not until Hokuto gives herself in Subaru's place that the story ends.
Subaru has his own intrepretation of this, which he explains to Kamui after Seishirou's death in X:
Subaru: Years ago... he almost took my life. But he didn't. I knew he could have found me at any time after that. He'd put his mark on me, you see.
Subaru: The seal that the Sakurazukamori use... to mark the prey that they will kill. At first, I didn't understand it. If he always knew where I was... why didn't he come finish me off? I thought about it over and over. And then I remembered the look... in his eye.
Subaru: The one he had at the exact moment when he decided not to kill me. It was... a look of utter scorn. As if I wasn't even worth killing. As if I were nothing to him.
This, Subaru explains, is what drove him to become stronger, in order to be enough of a nuisance to Seishirou to at least get the man to make Subaru's death wish come true. But — and this is the important part — Subaru also says that he must be wrong, in light of the words Seishirou said to him just before he died. He might not know if Seishirou was telling the truth, but it's certainly made a mark on him.
The point here is that Seishirou didn't kill him at the conclusion to the bet, despite the fact that it was a bet that Seishirou himself made, and furthermore a bet that was heavily in his own favor. That alone, I believe, is proof that Seishirou came to care for Subaru. It doesn't have to be in a romantic sense (and I don't think it was) — I can easily see Seishirou as simply being curious to see what would become of Subaru if he let him live. I think Seishirou, by that point, cared just enough to wonder what might happen if he didn't kill him. And it was enough to keep Subaru from dying at the end of the bet.
Subaru's interpretation is just that: his own interpretation. He's an incredibly biased source in this whole affair, but since Seishirou never actually talks about anything, we only have Subaru's words to go on. (Which may have been intentional, honestly; a lot of Seishirou is left up to the reader's interpretation.)
Now, obviously, the bet was favored for Seishirou from the beginning, but could Subaru have won if they were on an equal playing field? We know that by the end, Subaru loves Seishirou despite the fact that he's the Sakurazukamori and killed his sister, but if Subaru knew the stakes beforehand: could he have won? It's a hard question to answer. Subaru and Seishirou are two very different people, and while Subaru comes to understand Seishirou to the point of taking his role by the end of X, the child in Tokyo Babylon would have a much harder time coming to terms with the Sakurazukamori. Maybe things would have turned out the same way — or maybe they wouldn't. Who knows?