Disclaimer: Oniisama e and all characters therein belong to Ikeda Riyoko. I'm just borrowing them for nefarious (but nonprofit!) purposes.
Warnings: suggestions of incest (by marriage, not blood). Minor spoilers for late in the series.
Brotherly
His love for Nanako is completely brotherly.
He tells himself that whenever he's reading her letters, feeling her joys and sorrows more sharply than he has any right to, his thumb stroking over the careful, feminine characters.
He never had a sister before Nanako, never had a sibling of any kind, and the first time he saw her, it was curiosity that drew him in, a searching sort of resentment. This little girl with the big eyes and the sweet, pealing laugh was the one who had stolen his father away.
He hated her on principle even as he examined her from every angle, trying to figure out why she had been chosen over him. He went home that day still wondering why, and the question lingered in his mind, like bitterness lingering on the tongue long hours after the food has gone.
Even as her mother's painstaking generosity mellowed his feelings of hatred and resentment towards their family, there was still a strange sort of intensity in his feelings toward her, a subtle fascination that stubbornly refused to fade. An imprint of feelings that had passed.
And so when she came to him, when he had the chance to do something for her as her brother, he accepted, something inside him leaping at the idea. When the first letter came, he studied it for a moment, looking at his own name and address written in her handwriting, feeling something he couldn't quite identify.
The letters come regularly now-- a few times a month, sometimes more-- but there's still a feeling of anticipation as he opens them, wondering what she has written to him this time. Will it be more about Seiran, or about her difficulties with her best friend? Maybe it will be about the Sorority this time, or one of her classes.
She has such a fresh, pure outlook on life, and with each letter he feels he comes to know her a bit better. Her impressions of the world around her, her hopes and dreams, her worries about the future... Her letters always have a cheerful tone, but he reads between the lines to see the sorrow and pain, the inevitable hardships.
It couldn't have been easy for her, being chosen for the Sorority without the proper background or connections. He doesn't completely understand how women shape and enforce their social structures, but he has a fairly good idea of how it works. She is being shunned, he's sure, and possibly bullied, for her audacity in entering an elite group she should have no place in.
But she survives it, goes on in spite of it, and that endears her to him even more. Kaoru's persistence in the face of adversity is her best quality, he feels, and it is one of Nanako's best too. Though they don't seem it on the surface, the two of them are not so very dissimilar, and maybe it is Nanako's similarity to his lost love that makes her so attractive to him; mainly as a person but also, in a way, as a woman.
Perhaps it is because of this similarity that he wonders if there is a possibility she will fall in love with him, as Kaoru did. Though she doesn't really know him at all, he knows that isn't really a barrier to romantic feelings. She will construct in her imagination all the aspects of him that are yet unknown to her, and they will likely surpass the genuine article.
He plays out confession scenarios in his head, and a secret, hidden part of him wonders if they are for her benefit or his. He wonders if she would write it in a letter, her thoughts of longing spilling out into the page and forming into poetic words of apology; guilty admissions that aren't nearly guilty enough for the crime she has committed.
Or maybe she will come tell him in person, brave girl that she is. She will come to see him at his apartment, or arrange for them to meet for tea; then, tears falling from her downcast eyes, she will cry out her feelings for him in a desperate, choked tone.
Though the scenarios vary, one thing remains the same: his rejection. He always lets her down gently but firmly, trying to spare her feelings while making it clear he can't return the sentiment. He tells her that he's in love with someone else, or that she's too young (Hah!), or that he can only see her as a little sister. He doesn't tell her the truth about her stepfather-- he promised her mother not to, after all--but it's believable enough all the same.
He consoles her, sometimes, tells her she's a sweet girl, incredibly cute, going to make some man very happy someday, though he knows the words, coming after a rejection, won't mean much. They will have the appearance of platitudes, ritual words, when in fact they are heartfelt, and the irony of that never fails to strike him.
He expects that, after such a failed confession, her feelings for him would either fade entirely or be altered into a more appropriate shape. He doesn't really think about that part of the scenario; it saddens him somehow, though he tells himself it's out of a normal human need to feel desired by others. He concentrates instead on the fervent tone of her voice as she speaks the words, the raw feeling conveyed in those few short syllables. It is a powerful thing, and he almost feels humbled at the thought of it being directed at him.
Once in a while, he wonders if she fantasizes about confessing to him as he fantasizes about receiving her confession. He supposes that's one thing he'll never be able to find out, even reading between the lines as he does. He supposes that's as it should be; a good brother wouldn't expect to hear about that sort of thing, now would he?