Android 21 is the little sister of No.17 and No.18 of the Red Ribbon Army.Dr.Gero kidnapped her when she was only 16 yrs old.He somehow mechanized her body so that she was able to have the ability to shape-shift into whatever or whoever she wanted.After knocking out Gero,she shape-shifted into him to fool No.19 to think she was him in order to escape from where she was.

Android 21 is the adopted sister of 18 and 17,sorry,forgot to say that.She was adopted because the mother of 17 and 18 thought she looked like 18.17 is super protective of 21,real sister or not.
21 is more bloodthirsty than any other android ever created.She was programed to kill Son Goku by any means possible, including torture.''21 will always be the most bloodthirsty android,she is undefeatable,literally.'' Is what Gero said to 17 and 18 before 17 killed him.

Watch out for 21 guys,or you might be her next victim!

17,18,and 21 wern't alwas blood-thirsty killer androids.They actually used to be sweet and loving.They all were brother and sister.17 is a boy with shoulder length black hair,a black shirt with a skull on it,ripped jeans and a orange bandanna around his neck.18 is a girl with a long sleeved stripped shirt,a jean vest,and a denim skirt with black leggings.21 is their little sister.She has blue-tinted shoulder length black hair,a pink shirt with a jean vest,and black leather pants.They were all good intill Dr.Gero came along.

He kidnapped 18,17 and 21.He mechanized the oldest pair first,then the smallest.21 can shape-shift,has incredible speed, agility,flexibility,strength,and ki(energy) power.Her brother and sister have strength,speed,agility,and power too.So you see.They wern't always bad...always.16 is a totally mechanical android. Dr.Gero made him out of nothing.He was a type of android that was too peaceful.He was created to kill Son Goku for Dr.Gero.

So there you have it!That's 17,18,21,and 16.