Kurayami was born with a body that was unnaturally nimble, and because she couldn't move like normal healthy children, her alcoholic parents called her useless and abused her, then ended up selling her to slavery for good.
The little girl was turned into a rubber toy for the kids of a noble family, however, didn't last long in that house as she was kidnapped by the ninja clan Nankotsu, whose leaders wanted to make her the mascot of the clan. Nankotsu meant "cartilage", referring to the training system of its ninjas, which focused almost entirely on flexibility, and so Kurayami was a dream-like mascot to have for such a clan!
Although her main motivation for ninja training in the clan's dojo was to find and kill both of her parents, in reality she was too weak and not allowed to go on missions. She didn't need any contortion training at all, being the most flexible living being in the clan and most probably in the whole country! But a lot of muscle training was required to be able to even stand up and move normally.
Training was very difficult for her, since it felt a lot easier and more comfortable to just lay somewhere, folded in half or more, which her body seemed to love the most. Standing up and walking was quite uncomfortable, but she tried to do that as often as possible, and even tried to learn dance and acrobatics in hope that one day she will be ready to go on real missions...
But Kurayami is not a clawless cat, she is well trained to handle her two short katanas in case of true emergency, though more regularly they're used to entertain the clan's leader and other members with some dancing and acrobatics.