Disclaimer: All characters mentioned herein belong to Jo. I'm just borrowing them.
Harry can't help but wonder what other people think of her.
Indeed, for the longest time he had seen her only as Ron's younger sister, which scares him a little bit, because while that is accurate in that she and Ron are siblings, it is such a small part of who she was it is nearly irrelevant. Classifying Ginny as merely "Ron's little sister" is a little like calling marriage "two people living together." Technically true, but only in a vague, non-specific way that misses the most important parts.
It scares him that he could have been so blind to her for so long. Not just because he's worried that he's missed out—that he could have been dating her for years, and the fact that he hasn't is some great cosmic joke—but because it also makes him wonder how many other people he hasn't been seeing properly.
He first started looking at her in a different light in his fifth year. He hadn't really noticed at the time of course- he'd been too busy having nightmares and being tortured by Umbridge.
And perhaps that's how Ginny was able to slip under his radar, seamlessly making herself a constant in his life while he was paying attention to other things, because suddenly he was openly confessing his worries about Sirius to her in the library over Easter eggs- which, now that he thinks about it, would never have happened in his fourth year. He never thought he'd be grateful to Umbridge for something, even in the circuitous, indirect way in which he accredited her to his romantic exploits with the youngest Weasley.
In this way, he will always be forever grateful to Michael Corner (Dean Thomas has not been extended this same gratitude). For while it may have been Hermione that advised Ginny to be herself in front of Harry, he doubts she would have been able to do that if she hadn't actually had a boyfriend. A legitimate boyfriend, who took her to Hogsmeade and wrote her letters.
Harry always gets stabbing headache somewhere just above his left eye when he considers the fact that he and Ginny were only actually a couple for 25 days, and that they never got to do anything like that. It generally gives him the strong urge to punch something, so he does his best to think about other things.
In that summer before his sixth year, she truly became a part of his life, his circle of friends, so much so that he truly missed her when school started up again. He wishes he'd seen it then.
It took him an absurdly long time to remember that one of the things that he'd smelled in the heady fumes of Amortentia had been Ginny's scent— and by "absurdly long," he meant until the next time he had come across the love potion and for a scant millisecond believed that she was in the room with him.
That was last week.
In sixth year she was a thousand things to him—"Friend," "Teammate," "Crush," "Girlfriend," "Comfort."
But now she can't be any of these things. Not just because he must cut himself off from her, for her own protection, but because now he finds she simply defies definition.
So now when he thinks of her, there is no title that pops up, no category for him to file her away under. She is just Ginny, because somehow, by some odd cosmic miracle, her mannerisms, her beauty, and her spirit all managed to translate into the single word of her name.