that not realy a spoil, since it's something that normaly already explain at this point, but to be sure =) :
in her past life, wu xie is keep imprisoned, trained (in medecine) and used as a guinea pig by her (fool scientist) grandfather in his research, one of them was to incorporate another soul (the black cat one) with her own, making human experiment in front of her, making her participate etc...
she suffered a lot all along, until she killed him, burning him with is research manor, and became free (after that she become veterinary and joined a underground organisation as a doctor (until her dead in a explosion)
you can say that at the start of the serie, she is totaly emotionaly broken, and feel nothing like human friendship/family/love/etc... she alway have a impassible face and never express herself because of that, and it's impossible for other to know that she thinks/feel (i realy like how they make her insensible face in the webcomic ^^)
she will get some emotion with time, little by little, the first one is family, because it's the first time she has family member that love, trust and take care of her, so she go from "i will survive" to "i want to survive and protect"
and.. broken like she is... she is ruthless, cruel and never hesitate to broke, torture and kill her enemys
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)