Android 17 Storyline (Dragon Ball Z Series)

Pre History
A young man whose past is unknown, his body was augmented by cybernetics by Dr. Gero. Together with his twin sister, Android 18, and the full android known as 16, Android 17 was charged with the task of killing Goku. However, 17 wasn't following orders when following through with this; he only wanted to kill Goku because he was bored.

17 would not be able to complete his task, as he was absorbed by Cell and killed when Cell self-destructed. 17 was wished back to life with the Dragon Balls, with the other people who had died at Cell's hands. Android 17 decided to live his life out as a mountain hermit, occasionally robbing people as a source of income.

Many years later, Dr. Gero met Dr. Myu in Hell, and the two created a duplicate 17. The two 17s merged bodies and became Super Android 17. 17's own sister assisted Goku in destroying the two 17s once and for all.
Android 17 is Android 18's brother
Before their cybernetic augmentation, 17 and 18 were twin siblings.