Kagero is the only female warrior of the Koga Ninja that is sent to investigate the mysterious deaths in Shimota village due to an even more mysterious plague. Her main job is as a "poison taster" for her master's food. It turns out that she is totally immune to poison, due the fact that her blood itself is a deadly poison. Merely touching her could be fatal.
Because of this, throughout her existance she has been thoroughly abused both mentally and physically, by all but Hanza and a few other Koga Ninja. Tessai's attack upon her is but one of the denegrating events of her life.
Due to the fact that she is a female warrior, and her blood is poison, she has never been treated with any respect... just as an expendable warrior. Because of this she views herslf as of no worth and is on a crusade to prove her worth, while proclaiming her readiness (and sometimes willingness) to die.
Fortunately Jubei treats her the way she deserved to be treated, as an invaluable friend, warrior and a beautiful woman. As shown by her facial expression, his seemingly cold words opened her eyes to a world she had never seen before.
This insight to Jubei's feelings is mutually recriprocated as Kagero falls for Jubei in a big way, and they are locked into a doomed love. For the first time in her life she truly meant something to someone.