Ugh

back of a sword fighter. although i do prefer her without it.

well...
talking about the back of a sword fighter,
it seems something wrong with the distribution of muscles,
but I can't put my finger on it.

The biggest issue is that the artist invented most of those muscles under the left shoulder blade. The ribcage is also a little too tall, and a tad too wide at the bottom.

This reminds me of all those renaissance artists who used male models for EVERYONE, including women and children. The results were confusing, to say the least.

tbqh this is what she sould look a little more like.

There's no reason for her to be as scrawny as she is, and I lol'd at that part of the story where she said she had a muscular body.

There are quite a few anatomy flaws here, but just because she's supposed to be "muscular" doesn't mean she doesn't have any curves. She's not a body builder, her build should be more athletic if anything. Slightly narrower shoulders and wider hips would help fix the issue of her looking like a man.