Oh wow. This... this was amazing. The bond between Keith and Red is masterfully done, and it really explains why Keith throwing himself out an airlock made her rescue him.

This is beautiful, both in concept and in craft. These were my thoughts as I read:

I thought the knives only transform once, when they're "awakened" at the Trials? We haven't seen Keith's transform back and forth since then, have we?

I love the line, "a vague shade of peace mingled with the shadows of questions he is unprepared to pursue".

"her give and take" might be better as "her giving and taking"

There might be a word missing from "bright scarlet and streak of sky"

"as they do" might read more clearly as "as they always do"

The ending is wonderful, with Keith showing how that moment felt from his point of view.

Overall, great job on this. :)