Quote Originally Posted by Yukimura
But I guess that's exactly what a tool isn't it? A person willing to give up their own desires and aspirations in order to server the desires of the person who was there for them at a miserable time in their life and took care of them.
No, a tool is someone who has no other desires or aspirations other than to serve someone else. Big difference.

Shizune is Dan's (Tsunade's BF) niece. Dan's sister (probably Shizune's mother) died in the war (and he was possibly taking care of her after this). When Dan later died and Tsunade left the village she took Shizune with her as her apprentice, they traveled together until Tsunade came back to the villiage to become Hokage.
As far as I know this was only mentioned in the english dub so I don't consider to be a part of the official canon, although it is a sufficient explanation.