Android 17 (人造人間１７号 Jinzōningen Jū Nana-Gō, lit. "Artificial Human No. 17") was originally a normal human named Lapis (ラピス Rapisu), but was transformed by Dr. Gero along with his twin sister Lazuri into Cyborgs (incorrectly called Androids in the dub). His programmed purpose is to destroy Goku with Android 18. However they are able to defy their programming and destroy their creator, and while pursuing Goku at first, they eventually abandon their programming completely.
While looking for Goku to kill him as per his programming, he is found and absorbed by Cell to help achieve his Perfect Form. After Cell's death, Android 17 is brought back to life along with Cell's other victims with a wish on the Dragon Balls. He abandons his mission to kill Goku and eventually becomes a Park Ranger, protecting wildlife on an island, and starts a family. In Dragon Ball Super Goku finds him to join the Universe 7 team in the Tournament of Power. Since the Cell games 17 has been training and became far more powerful than he originally was.
In Dragon Ball GT another Android 17, created by Dr. Gero and Dr. Myuu in Hell, comes to Earth and fuses with the original 17 to become Super 17.
As may be expected of twins, Android 17 is similar in character to his sister, Android 18. 17 is seemingly more mischievous than 18 though, having been the one to suggest most of their sinister acts after being introduced to the series. To him, the world is his playground, stealing cars when he could more quickly fly anywhere, and destroying entire cities for the fun of it.
In Dragon Ball Super 17 seems to have become a kind-hearted person and values life. He still enjoys a good fight but takes himself more seriously, not wanted to leave the wildlife on the island he guards unprotected.