She walks down the stairs of the school, and in that moment, he both knows and doesn't know that he loves her. He swells with a feeling so unfamiliar to him, but somehow, though he is transfixed by the blonde girl, he places the feeling as something akin to what he felt towards his little sister. Love. He is in love with this tiny girl.
It's not because of her long hair, or her short skirt, or the lollipop that she's swirling around in her mouth (although he does like that). It's because he sees the life twinkling in her eyes, and how the only sun that doesn't burn him is blazing in her heart, and he knows right then that he wants to stand close to that sun and protect that life. He can see that she's going to become so much more than a vapid high school girl and he knows that she can show him how to be more than what he is now.
He hadn't even properly met her, and she's already changed his life. He loves her; of course he'll help her.
(and secretly, he really likes that skirt)
He's not sure how long he's been here. Whatever Hell dimension he's in, the lack of day and night, coupled with his lack of aging, make it impossible to tell the time. He doesn't care, though—he deserves the timeless pain, he tells himself.
His demon screams and rages inside his head, shouting profanities directed at the Slayer, telling him that he should be plotting their escape, and ultimately, the blonde girl's death. Despite what his demon says, though, he doesn't hate her. She shoved a sword through his gut to save the world from him. He fell in love with the one person who can stop him, and he loves her all the more for it.
(there's pain again, and oh, there's another sword in him, and his demon finds it absolutely hilarious that it's through his back)
He can feel her eyes on his back as he walks away, and he's never felt pain like this before. He's spent enough time alone these past months, running over every one of her expressions in his head that he knows exactly which one is on her face now.
(but what if it's a new one, one he hasn't seen, and he almost turns around to check because he has to remember everything about her if he's going to do this)
He keeps walking.
They both know that she could do something other than watch him go—she could shout his name, or run up to him and make him stop. And she would ask him to please stay with those big green eyes, and he would because he hates this so much, and they'd be (somewhat) happy for a bit, until something happens and he'd have to go away again for good, and he'd sneak away while she wasn't around this time, and the extra months together would just make it that much harder.
But she stands still and he walks away. She keeps things as painless as they can be in this situation, and he loves her for it.
(so this is what heart ache feels like)