Tesla

Tesla was an independent Plant, and is also Vash and Knives's older sister. In the manga, she is the main reason for Knives breakdown towards humans.

Tesla was the first known independent plant to be born, and as such fell victim to becoming a test subject under humans.

She appeared to Vash and Knives as a vision that lead them to the medical block where the experiments on her were performed. Tesla was a small girl with blonde hair and a freckle on the left of her chin. She wore the universal outfit for children on the SEEDS ships. Barely anything is known about Tesla's personality.

The Report of Tesla was the recording of experiments performed on Tesla by the humans up until her death. Tesla’s body was preserved in three separate cylinders tanks after her death: one containing her right arm, another containing the majority of her dissected body, and the last encasing her brain and eyes.

(Source: Trigun Wiki)