A/N – Sesshoumaru did not think of Rin as human.
Disclaimer – Inuyasha belongs to Takahashi-sensei. I'm just borrowing one of her more intriguing relationships.
Flesh and Spirit
"…now and forever remember: you are flesh as well as wishes, body as well as spirit, and when you let one fly without the other, then look to suffer for it."
C. J. Cherryh, 'Fortress in the Eye of Time'.
Sesshoumaru did not think of Rin as human.
To him, humans were sheep; herd animals that stuck together for mutual protection and gratification, not venturing any farther beyond their safe, predictable boundaries than they absolutely had to. Anything they could not explain was enshrouded in magic, taboo and superstition, and anything different was feared, despised, or destroyed. They lived in squalor and backbreaking poverty, never lifting their heads above the mud and dirt of their fields, living all of their firefly lives concerned only with preserving their pathetically weak bodies, eating, and procreating.
There was no room for anything else. There was no room for dreams.
But Rin had stumbled – orphaned, despised, abandoned by her fellow humans – into a dream in her sixth year, and had welcomed it, embraced it eagerly. She followed him out of choice, because she willed it –
Do what you want, Rin
And in following him, she entered a world of dreams and nightmares alike. For every mindless, ravenous youkai who tried to devour her, there was a field of glorious flowers and a sky full of jewel-bright stars. For every mid-level youkai who despised her because she was human, there was a flight on Ah-Un's back, and a freezing winter's night curled up next to Sesshoumaru's own, much greater warmth.
For every danger that would have sent grown men screaming in fear, there was the sure certainty that she would be safe, that he would protect her. Under his protection, she grew farther and farther away from the bruised, frightened girl she had once been. She knew no fear, and no sense of human limitations – her spirit threw itself gleefully into her new world trusting that he would always support her.
Even Izayoi had doubted and feared. Sesshoumaru knew that his father's woman had been older than Rin, and less – or perhaps more – naïve. She had known exactly what Inutaisho had been, and of the consequences of choosing him over the security of her human suitor and family. Rin, however, had no loving family, and no close ties to humans, nothing to anchor her to the 'real' world except fear, which she discarded as useless after her death and revival.
No, Rin's spirit was more akin to his than to any of the fearful, shrinking, earthbound humans who might seek to reclaim her. They would draw her back into the world of flesh, hedge her about with fears, cautions, restrictions, and slowly strangle her. Sesshoumaru was largely indifferent to human suffering. They were nothing to him, a shifting, faceless horde – but he did not think of Rin as human. She was a wild spirit in a human body. Her upbringing had shattered the usual human conditioning, and set her free to follow him.
Rin wants to stay with her Sesshoumaru-sama forever!
And for the preservation of that spirit, so inadequately housed in such fragile flesh, he would allow it.