sister, friend, substitute
Hokuto is the best. Like, hands down the best character in this series. If I had more to say about her I'd be making a site about her, but instead I'm writing about a tragic pairing. Clearly I have made a mistake somewhere.
Anyway. From the very beginning of the series, Hokuto supports Subaru's relationship with Seishirou. At first it seems odd, given that she's also the vehicle for exposition and the one who tells us that Seishirou is maybe the Sakurazukamori, and in truth as the series continues it's clear that while Subaru might be falling for Seishirou's tricks, Hokuto isn't. She knows there's more than meets the eye. She knows he's hiding something.
It's not that she doesn't care about the danger. It's that she cares more about Subaru's happiness.
Case in point, from the "Save A" chapter:
Hokuto: Besides, this only proves you should be with Seishirou-chan! Subaru the prospective zookeeper and Seishirou the veterinarian... a match made in heaven!
Hokuto: And to top it all off, Seishirou is related to the mysterious Sakurazukamori assassin clan... which we Sumeragi still haven't quite figured out! It's like we're living in a shoujo manga!
Subaru: But obaachan said... not all people named Sakurazuka... are connected to the Sakurazukamori! And besides... I can't believe that scuah a kind person as Seishirou-san could be an assassin.
Hokuto: You're right... he does seem very kind. But... we don't really know... if he is.
(Obaachan is grandmother; Subaru is referring to their grandmother, the previous Sumeragi clan head.)
Throughout the series, Hokuto is fiercely supportive of Subaru. In the scene just before this quote, she tells him to never give up on his dreams, no matter how demanding work is. She supports him in everything, from his relationship with Seishirou, to his work, to his farfetched dreams. She is always, always there for him. Yes, she can tell that Seishirou is dangerous — but she can also tell that he's something that Subaru wants. And she can't help but support him. I'll bring this up again later, but I want to emphasize it here: everything Hokuto does in the series is to support her brother.
Hokuto calls Seishirou out when he avoids her questions, but she never questions him in any more depth. In fact, she's the one to threaten him: in the "Save B" chapter, when Seishirou asks if it's all right for him to pursue Subaru while they're cooking, she holds a knife against his throat, smiling all the while.
Seishirou: But... are you sure you're okay with this?
Hokuto: With what?
Seishirou: Well... Subaru-kun is the very picture of morality. Are you sure it's okay for me to seduce him?
Hokuto: If he accepts you. But... if you ever make Subaru cry... I'll kill you.
This is hugely important: her consent to the match is based on if Subaru accepts Seishirou. Subaru's happiness is what matters to her, and if Seishirou hurts him, then she will take matters into her own hands. (And, y'know, she does, but we'll get to that later.)
I think the most important thing to remember about Hokuto's relationship with Seishirou is that she really, truly had hope that they would work out. In the "Rebirth" chapter, she tells him about Subaru's emotions, and how if he were to fall in love and be betrayed, he would break, clearly foreshadowing the ending. It's not that Hokuto didn't see it coming, or that she didn't know that Seishirou was capable of it. She knew that if her worries came true, and if Seishirou was in fact as dangerous as she feared, she stood no chance against him. But that didn't mean she couldn't warn him that there would be hell to pay if he hurt Subaru.
And so, when Seishirou loses his eye, Hokuto can tell from the moment she walks into his hospital room in the "News" chapter that something has changed. More than that, she calls him on it, and tests him.
Hokuto:: ...I don't understand you. I always figured you had a flip side.
Seishirou: A flip side?
Hokuto:: Well, you always seem like such a gentle, smiling person. But that doesn't mean you're always smiling when you look the other way. Hey, I'm talking about you, Sakurazuka Seishirou-san!
Seishirou: What made you think that about me?
Hokuto:: Your eyes. You hide behind your glasses... but your eyes... they're not the eyes of a normal person. And... though you've hidden it well behind those lenses... I can smell the blood on you.
Seriously, Hokuto's relationship with Seishirou is incredible.
She asks him to promise not to take Subaru away, unknowingly echoing her grandmother's prediction from the "Dream" chapter, but they're cut off by a nurse before Seishirou can respond. Still, it's enough for Seishirou to realize that his time is up, and that he can't keep playing at the bet with Subaru anymore; it's time to call it in. So he does. And just as Hokuto predicted, Subaru breaks.
Afterwards, Hokuto comes to Subaru, who has gone within himself, and shut out the outside world. Hokuto blames herself for what happened, and it's important to note that while Hokuto may have made it easier for Seishirou to get close to Subaru, she is of course not at fault for what happened. Seishirou's the one who made the bet in the first place and set this whole thing in motion; Subaru and Hokuto are victims.
She does explain her mindset for why she let Seishirou get close in the first place, and again, while she's not at fault, I think it offers a lot of insight as to why she would let someone she clearly knew was dangerous get close to her brother.
Hokuto: Subaru... you're always so gentle and honest. But... you absolutely never think of yourself! Even though obaachan and I need you so badly... you don't realize it at all. You truly don't think it doesn't matter... if you die. That's why... that's why I wanted you to become devoted to something. You always said you wanted to become a zookeeper. That you gave that dream up for the Sumeragi clan. But if you could give that up for the sake of others... it couldn't have been something you were truly devoted to. I wanted you to have something you would love so much that you wouldn't care what others thought... something you wouldn't change your mind about. It didn't matter what it was. I just wanted you to have something like that. That's why I brought the two of you together. Because I knew from the very beginning... that he would become... special to you. I knew he was dangerous... but even so... I wanted you to truly live! You're human too, Subaru. I wanted you to realize that you had a soul. But I never wanted things to end this way!
Seriously, this is heartbreaking stuff. Hokuto loved her brother so much, and knew him so well, that she could tell that he needed something to love. She just didn't realize what would happen if Seishirou really did turn out to be the Sakurazukamori, or if things turned out badly. Was it naïve of her? Probably. But it came from a place of genuine love, and I think she was taken in by Seishirou's lies as much as Subaru was. And when the truth came out, it was too late.
So she decides to take matters into her own hands.
Now, in Tokyo Babylon, Hokuto's spell is vague at best. Within the actual "End" chapter, we only get a glimpse of it, and the rest comes in "Annex/Start." (I could talk all day about how great "Annex/Start" is, holy shit. What a perfect ending.) In X, we finally learn that her spell was to make it so that if Seishirou tried to kill Subaru as he killed Hokuto, it would rebound on him. I've discussed her spell some elsewhere, but what I want to talk about here is Hokuto's purposes in casting it in the first place.
Hokuto: I'm putting my trust in you that you will never... set this spell into motion.
Seishirou: I'm not the kind of man you should put any faith in.
Hokuto: I know that. But... you still... mean so much to Subaru. So I want to trust you. And I want you to remember one thing. You may have committed sins you can never atone for... but no one is out of love's reach. Not even you...
It's not that she was making good on her promise to kill him if he ever hurt Subaru; far from it. It's that she didn't want Subaru to die. She says as much in Tokyo Babylon when she went to face Seishirou, that she wouldn't allow him to kill Subaru. Sure, one could argue that all Seishirou had to do was kill Subaru through any other method than a hand through the chest to avoid the spell, but this is CLAMP so logic doesn't apply. (Also, as discussed, again, elsewhere, being killed by the one he loved is exactly what Seishirou wanted anyway. Thanks Hokuto! Wait this is the opposite of what you wanted.)
Unfortunately, and I think this is the true tragedy of the series, not the bullshit Kakyou was spouting about "oh boohoo I knew you were throwing your life away but I couldn't do anything" — what I don't think Hokuto realized is that ensuring Subaru's life didn't really put him in a better position than the one she left him in. Let's remember, here, that when Hokuto last saw her brother, he was comatose over being betrayed by Seishirou. She then left to find Seishirou and be killed by him, and cast a spell on Seishirou. Nine years later, Subaru kills Seishirou due to Hokuto's spell. He's still left, uh, without Seishirou. As thoroughly established through both series, the key to Subaru's happiness is either being with Seishirou, or being killed by Seishirou. Hokuto's spell ensured that the second couldn't happen, and the first was only possible if Seishirou got over himself and decided to make nice with Subaru. And look how that turned out.
Don't get me wrong, I like how the series went, but I do think that ultimately Hokuto's sacrifice put everything on Seishirou's shoulders and made it his responsibility to make things right, and, uh, given how well that went with the bet, I don't think that was a good choice to make. I like that Hokuto tried to get through to Seishirou, and that she trusted him, but I also don't really think things could have turned out any other way. Maybe she should have tried taking Subaru to therapy instead.
One last thing I want to touch on is Subaru in "Annex/Start." At the beginning of the chapter, after the dream of Hokuto's death, Subaru looks in a mirror and remembers a conversation he had with Hokuto when they were younger, about being twins and being considerate of each other. Hokuto had wanted him to say good morning to her every day, and even after her death, Subaru says it, and then breaks the mirror.
At the end of the chapter, Subaru talks to the ghost of a young girl.
Subaru: The place where you live... is my sister up there?
Ghost: Dunno. What's she like?
Subaru: She's just like me.
Ghost: Then she's not up here. ...There's no one up here with such lonely eyes as yours.
Now, there are lots of ways to interpret what the ghost says about Hokuto, but what I want to talk about is actually Subaru's response: she's just like me. Here's the thing: the scene at the start of the chapter clearly shows that Hokuto is not just like him, and they had a system in place to take care that they would be considerate of that very thing. Subaru has cut his hair since her death, he's started smoking, he wears plain clothes and not the colorful outfits Hokuto used to pick out. He's very different from the Subaru he used to be, and even further removed from Hokuto.
Hokuto understood her brother very well. Most of the insight we get into Subaru's personality during the series is from her. However, that doesn't go both ways. I don't think Subaru understood Hokuto all that well, and I think this quote makes it really clear: even after she's gone, he still thinks that they're the same, when they've never been. They're different people, and always have been.
Would things have gone better if Subaru had understood his sister better? Maybe, maybe not; it's hard to say. I do think, though, that while Subaru obviously loved his sister very much, things probably would have been easier for him if he'd opened up about his feelings for Seishirou and confided in her much earlier, rather than letting them bottle up until the worst moment possible.