Derris-Kharlan is immaculate, because Lord Yggdrasill wouldn't have it any other way. Sparkling spires towering into what passes for the sky; floors one could literally eat off of, so rarely are they walked upon; air unsullied by the smokes and vapors of the world below -- all maintained by the labors of angels to whom it provides no benefits, immune as they are to illness. A perfect world, to the eyes of some.
Kratos watches a robot patrol the lonely halls, sucking up the stray feathers that chanced to fall, and wonders if this is really what Mithos wanted. It isn't what Martel wanted, certainly. His mental image of her is of her dirt-smeared face and hands, smiling happily as she shows him a newly-planted tree. But Mithos had never liked mucking about in the soil, no matter how beautiful the flowers he planted would have been. They would have died within a season, and the dirt would still be there.
Sometimes when he goes to Yggdrasill's rooms, he finds him at the sink, scrubbing at his hands with a feverish air, seeking to eradicate microscopic specs of dust only he can see. He stands there for as long as half an hour, until his hands are worse than when he started, red staining the white basin only to be washed away by the stream of water. Kratos never says anything, just casts a healing spell; and Yggdrasill says nothing, merely lets his perfect hands fall to his sides; and nothing changes, because they both know it'll happen again tomorrow.
Derris-Kharlan is immaculate, and yet somehow Kratos thinks it couldn't be any more unclean.
He looks at his own hands, sees the red staining them, and decides to let them be. They'd just get dirty again anyway.