an eye for an eye means everyone's blind
Losing one's eye has become a cliché in CLAMP manga to the point that you can almost expect it to happen, making the actual event completely anticlimactic. (I'm looking at you, Tsubasa.) At the time Tokyo Babylon was written, however, it had yet to become a cliché. Either way, losing one's eye is a huge issue for both Seishirou and Subaru.
In Tokyo Babylon, Subaru gets to know a young boy called Yuya who has kidney disease. Subaru learns that Yuya's mother had had a daughter with the disease, and had the option of donating one of her kidneys to one of her children. She chose her daughter, who rejected it and passed away shortly afterwards, and as such she is desperate to find a kidney for Yuya. Her desperation eventually leads her to attack Subaru in an attempt to make him give Yuya his kidney (tragically, just as he was about to offer his kidney to the child anyway). Seishirou steps between the woman's blade and Subaru, losing his right eye in the process.
Subaru is devastated by Seishirou's loss, and results in a way we've never seen him — screaming Seishirou's name, crying uncontrollably, and so forth. He blames himself and is terrified Seishirou will hate him for it, but Hokuto manages to get through to him and tells him to go apologize to Seishirou. To Subaru's surprise, Seishirou doesn't blame him at all.
Subaru: I'm sorry...
Seishirou: What's going on? Why are you apologizing to me, Subaru-kun?
Subaru: Your eye... Seishirou-san...
Seishirou: Oh. You mean this? But why are you apologizing, Subaru-kun?
Subaru: It's my fault... you tried to protect me...
Seishirou: That's not your fault, Subaru-kun. It was my choice.
Seishirou goes on to explain that he doesn't blame Subaru at all for what happens, and that his intentions were entirely selfish. Given his true nature, this seems strange at first — even with the bet, why would the heartless Seishirou want to protect him? The true reasons, I think, are layered in a few different things.
Firstly, the main theme of Tokyo Babylon is that each individual person's choices are their own, and no one else's: to quote the line that opens this chapter, There is no such thing as "everyone." It thus follows that Seishirou's decisions, fucked up as they are, fall under that same philosophy. Subaru can't judge him or scold him for them. Secondly, Seishirou is acting based on the bet. The reasons he made this choice are tied up in something that Subaru isn't aware of in the first place.
And thirdly? I think Seishirou is just plain possessive.
Seishirou: You never asked me to protect you... or to do something about Yuya-kun's mother. Everything I did, I did by my own choice. You're not responsible for anything, Subaru-kun.
Seishirou: Everyone's actions are entirely selfish. Even when we want to do something for another person... it's only because we want the satisfaction that we made someone else happy. My actions were no different. You were prepared to let that woman seriously injure you. I preferred you to be healthy. So I got in her way. I was being selfish... nothing more.
The deception Seishirou shows to Subaru through the year of the bet is only as good as Seishirou himself is, and this logic is flawed by Seishirou's own mindset. Seishirou himself is a selfish man, so he believes that he's in the right with this thinking. Furthermore, shortly after this, he declines to press charges against Yuya's mother and says he really doesn't care about it — hardly the reaction of someone devastated by this kind of accident. Whether or not he was lying to Subaru, I think that Seishirou genuinely didn't care.
Subaru, on the other hand, never gets over it. In fact, it's this incident that causes him to realize what he truly feels for Seishirou. The "Pair" chapter, which I feel is the perfect prelude to the ending, showcases Subaru's meeting with a blind man who has a seeing-eye dog. The man explains that the dog acts as his eyes, and if Subaru wants to help Seishirou, he could be his eyes. In talking to him, Subaru realizes his feelings, and goes to confess, and, well. We all know how that turned out.
The issue of losing eyes comes up again in X, again involving Subaru and Seishirou. Subaru defends Kamui against Fuuma, and during their battle, Subaru can't help but wonder why Fuuma seems to resemble Seishirou, and is distracted. Fuuma takes the opportunity to stab him in the eye.
Just as Subaru blamed himself for Seishirou's injury, Kamui blames himself for Subaru's. But like Seishirou, Subaru is unconcerned, and admits that it was what he had wished. Here, Kamui and Subaru take on the roles Subaru and Seishirou played in this same tragedy nine years prior. But there is another concerned party in this case, and that is Seishirou. At the time Subaru loses his eyes, Seishirou seems impassive and unconcerned. In truth, however, he is anything but impartial, though he never shows it to Subaru. After Seishirou's death, Fuuma brings Seishirou's left eye to Subaru and tells him how Seishirou really felt.
Fuuma: You lost the eye. And... the Sakurazukamori wasn't too pleased about it. He... couldn't stand the idea of you being scarred by anyone else but him. And so, what he desired most was to erase that injury.
Subaru accepts the eye, and in so doing, inherits the Sakurazukamori's powers and takes Seishirou's place among the Dragons of Earth. In truth, though, this is but the last step to complete Subaru's becoming Seishirou. I don't mean in a literal sense, of course, but figuratively, everything that made Subaru, well, Subaru is now gone. The only thing he has left is the desire to keep any part of Seishirou still with him, since he can't have Seishirou himself. If that means becoming the Sakurazukamori, so be it.
But it's not like he hasn't been on that path since the end of Tokyo Babylon. Subaru was a kind, loving child who threw away everything in pursuit of Seishirou. He took up smoking and closed his heart away, refusing to get hurt again. He was turned himself into an emotionless shell in order to chase down another — and when he found him, the two of them were no better at getting through to each other than before.
More than anything, though, Subaru became selfish, focusing only on his wish. Time and time again, he told Kamui that Seishirou was the only one who mattered to him, and that his happiness laid with him alone, even though it would hurt those around him. He encouraged Kamui, in turn, to find the wish that made him happy, no matter what others thought of him. So, obviously, when Subaru kills Seishirou and ends up taking his place, he betrays Kamui just as Seishirou once betrayed him. He had no choice in Seishirou's death as a result of Hokuto's spell and Seishirou's decisions, but he ultimately makes the choice to take his place.
- an eye for an eye
- in law and custom, the principle of retaliation for injuries or damages. In ancient Babylonian, biblical, Roman, and Islamic law, it was a principle operative in private and familial settlements, intended to limit retaliation, and often satisfied by a money payment or other equivalent.
So both Seishirou and Subaru lose one of their eyes during their lives, and the wound is more than skin deep: their injuries and their ramifications also have a figurative significance. It is through the loss of Seishirou's eye that Subaru comes to realize his feelings for Seishirou, which he had been blind to previously. He was also completely blind to Seishirou's true nature, unable to see beyond the façade Seishirou presented to him. Given the fact Seishirou had erased his memory of their bet and never showed him anything but kindness, one can hardly blame him, but that does not change the fact that he was oblivious.
This blindness is even more apparent in X. Seishirou and Subaru both think they know what the other person wants: Seishirou believes Subaru wants to kill him, while Subaru only wanted to be killed by Seishirou. Even if they had had both their eyes, they would not have been able to see what the other really wanted, and so things were doomed to their tragic conclusion. Perhaps it could have been been avoided, as Hokuto wished, but it was not to be.