Created about a decade ago by the AI inside Red Ribbon Army's Doctor Gero's hidden supercomputer by combining the cells of Son Goku, Vegeta, Piccolo, Freeza, and Majin Buu, Android 21 can be considered a more focused and streamlined attempt at creating the ultimate lifeform than the mighty Android Cell was. Using more recent DNA samples of less but way more powerful fighters proved a success, and the addition of Majin Buu's cells skyrocketed Android 21's raw power and abilities to levels the Perfect form of Cell wasn't even able to give birth as mere imagination. Not only Android 21 possesses the skills of most of the greatest fighters in Universe 7, but more than enough power and the IQ of a supergenius to fully use them, with the regeneration factor of Majin Buu added in to make sure Android 21 is, under most circumstances, unkillable. With everything set in place, she is (at least in theory) the ultimate lifeform Doctor Gero's computer hoped for. But is she really a perfect beign?
The answer is a big no. Power, intelligence, and abilities are present and working just as expected if not better, but the addition of Majin Buu's cells would eventually cause unexpected problems of various sorts over Android 21's mind that strayed her from the originally intended perfection she was suppossed to achieve. Her Majin nature took over as dominant, thoughts became erratic and unpredictable, even childish, and even now, she's often overtaken by the same immense urges to destroy and consume the original Majin Buu had. Her only thread to sanity comes and goes, and has developed a multiple personality disorder not uncommon in the Majin. Android 21 can switch from just playful yet approachable to a monster trying to consume even those she considered friends a second ago and every personality inbetween with little or no trigger at all, so extreme caution is recommended. Fueled perhaps by her Freeza's cells, she once thought about a grand scheme to gather and devour Son Goku along with the Saiyan's friends and enemies, but in the end the lazy and volatile nature of her most Buu part made her quit the idea before any step of the plan was done. Not bothered by the red lines of right and wrong or good and evil, the defective and too powerful for her own good Bioandroid does as she pleases, when she pleases.