Wow, another HL/Firefly crossover, and this one even more unexpected than the last. Lovely.
Read
Constants, by Rhi.
They are Companions
who know how to kill, thanks to her, and how to live, thanks to her, and
how to endure when they'd rather die, and how to die if they must not
live... all thanks to her. They never speak her name outside the chapterhouses,
even amongst themselves. They never speak of where they learnt such skill
with a blade, or such timing with explosives, or the fine art of knowing
silence.