Android 21 may seem normal on the outside but on the inside, she is an entirely different case. Due to being a combination of multiple DNA, she has all the traits of the Z-Fighters, Cell, Frieza, and Buu in one package. However, the combination of cells in her body caused
an unanticipated reaction, leading to her to developing an excessive compulsion to eat which spiraled out of control and eventually constructed an alternate personality which turns her into a sadistic, power-hungry sociopath who only wishes to “devour" all the powerful beings. The longer she resists the urge, the more her evil side gains control.