Nanako works for Dr. Ogami Kyoji, shes clumsy among some of her other talents.. Dr. Kyoji doesn't seem to like her much though. Basically Kyoji wants to put her brain into a machine of sorts.. The anime is basically just fan-service which is a plus+ =D
The original Nanako worked for Kenji since the age of 16. After Joji was born, the original Nanako died there was no explanation as to why. Kenji created a cloning technology and in 1941 and cloned Nanako. When Nanako A turned 20, she died. Nanako was then cloned again in 1962, Nanako B. Nine years later, Joji had a son, Kyoji. In 1978, at age 7, Kyoji meets Nanako for the first time when she starts working at the hospital and becomes very attached to her. In 1982, Nanako becomes very sick and of course Kyoji isn't happy. He promises Nanako that he'll grow up to be the world's greatest doctor so he can cure her, but at age 20 she dies again. Kenji dies at age 74, so Joji clones Nanako once more and then retires in 1983. This Nanako is 16 in 1999 and begins working at the hospital with Kyoji. Kyoji figures out a way to stop Nanako from dying at age 20, a process using Green's triple helix DNA. All the training she does to make sure she is strong enough to make it through the operation.. but you don't find all of this out until the final episodes.