She reminds him of snow, usually.
Beautiful, pristine, perfect, and yet cold to the touch. Once in awhile, when they're all sitting around the open fire, he'll look at her and wonder if she can feel any heat through the icy layers she wraps around herself. Every so often, when she stands alone in her silent requiem, he'll play little games in his head: which façade is it today, has her forehead crinkled in thought, how many times has she sighed, and is that smile real or fake? Sometimes he wonders why he wants to know, but mostly he finds himself just musing about the snow and all the little intricacies he doesn't understand.
Occasionally, when he can see the tears she is unwilling to shed, he panics; he wants to peel away her skin and peer inside her head, wants to make sure that a spider hasn't gobbled her ice heart before he can melt it.
Frequently, repeatedly, he pries her ice cage open with bare hands, just to see if she's still there. Always, she slaps him.
At those times, Sango reminds him of fire.
And in those moments, Miroku remembers why the pain is worth it.