Nightblade by Ryan Kirk.
Here is the description from Amazon:
Ryuu is a boy orphaned by violence at a young age. Taken in by a
wandering warrior, he learns he may have more strength than he ever
imagined possible. A quiet child, Moriko is forced into a monastic
system she despises. Torn from her family and the forest she grew up
in, she must fight to learn the skills she’ll need to survive her
tutelage under the realm’s most dangerous assassin.
and destitute, Takako is sold to pay for her father’s debts. Thrust
into a world she doesn’t understand and battles she didn’t ask for, she
must decide where her loyalties lie. When their lives collide together
in a Kingdom on the brink of war, their actions will determine the
course of history. If they can stay alive.