Due to her biology, Android 21 has a natural, compulsive instinct to feed (which grew out of control due to cells in her body going berserk). Like Majin Buu, she prefers sweets and confectioneries, her favorite being macarons. However, she has no control over her hunger and constantly fights to keep herself from turning others into treats and devouring them.
Her hunger is so great and painful that it has led to Android 21's psyche being damaged and manifesting a split personality. The longer she goes without feeding, the more the evil side of her gains control. When her true persona is in control she acts like a kind and motherly woman who was concerned about Android 18 and agreed to help her. When her evil persona is in control, however, she becomes a sadistic and power-hungry sociopath, wanting to "devour" all the powerful fighters, drawing to the point of near insanity. Her gluttonous lust for "devouring" powerful fighters is similar to the traits of the Majins, inherited when Buu's cells were added to her. Her good side is friendly towards the good Majin Buu presumably due to their shared biology and love of sweets.
Her personalities were later transferred to her two halves after her good self-managed to use fission as a result of the emotional turmoil caused by the death of her adoptive son, Android 16 who was killed by her evil side as a result of her losing control.
Though Android 21's good side is aware of the fact that her human base was the mother of Android 16's template which causes her to develop mother-like affection for him, she admits she has little to no memory of her former life as a human, though she enjoys thinking about what her original name and life might have been like. She is also apparently unaware that Dr. Gero was the father of Android 16's template or of his relationship with her human template. During one of her conversations with the player, she notes that her creator while not a good person was a brilliant scientist and admits she would like to know the reason she was created though admits that he might not answer her questions or that if he did she might not want to know the truth. Interestingly she never mentions Dr. Gero's name directly though it is obvious he is the one she is referring to. Presumably, her knowledge of him either comes from what little memories she possesses and what she has been able to learn through research of herself and Android 16.
In contrast, her evil side cares nothing for Android 16 and believes that Androids have no use for things like family and loved ones as she orders Android 18 to attack her husband Krillin. Additionally, she only values Dr. Gero for his labs and research. Though she is focused primarily on feeding, the evil side of Android 21's personality does not dismiss the concept of seeking galactic conquest though that goal remains secondary to satisfying her hunger. Unlike her good side, the evil 21 cares only about herself and possesses selfish bratty personality. Like Cell, her power causes her to view other beings as beneath her though additionally, she dismisses all other beings as little more than food to make her stronger. Though she is responsible for creating evil clones of the Z Fighters as well as Nappa, the Ginyu Force, Frieza, Cell (all of whom she resurrected with the Namekian Dragon Balls), and Kid Buu, they exist only as a source of food to sustain her and her hunger, despite finding their taste to be unsatisfactory. She only allows the Z Fighters, resurrected villains, and Androids to survive and grow stronger in order to further her goal of consuming them. However, she was angered when Z Fighters and the villains led by Frieza and Cell started hunting the clones to deny 21 of her food supply, causing her to seek them out in a blind rage driven by her hunger.
After separating from her good half, the evil 21 mocked her good half's refusal to feed on others to increase her power and was certain that she was a hypocrite who would eventually give into her hunger and feed on her allies. However, the good 21 was able to link with the human soul that had been linked to Android 18 which allowed her to maintain control over herself and oppose her evil half without succumbing to her hunger. Still, the good 21 feared she would one day lose control, and thus decided to sacrifice herself to stop her evil half as well as prevent herself from losing control and hurting her newfound friends. Whis and Beerus noted that in time the good 21 would have succumbed to the hunger, so her sacrifice was ultimately for the greater good. However, it is unclear if the good 21 could have overcome her hunger permanently, though she may not have wanted to take the risk of losing control of herself. The self-sacrificing aspect of the good 21's personality fittingly resembles that of her adoptive son, Android 16. did
""I think about it alot these days""
Being created from the cells of countless warriors and scientists, Android 21 is one of the most powerful Androids in the series and the most powerful female Android created though Dr. Gero's research. Like Majin Buu and Cell, she can increase her power by absorbing other life forms, which she does by turning them into sweets and eating them increasing her power. Additionally due to the cells of countless scientists she has a high IQ which my even rival that of Dr. Gero himself. This allows her to utilize Dr. Gero's research for her own purposes though her evil split personality and hunger make her dangerous. Her evil persona uses her intelligence and Dr. Gero's research to achieve her goals which presumably includes galactic conquest though it remains secondary to her impulse to feed which is her driving purpose. However her good personality uses her intellect to oppose her dark impulses. The evil 21 is very manipulative and uses her intelligence to manipulate her enemies and 16, thus she is more dangerous than Cell who while an intelligent fighter tends to rely more on his power, while evil 21 plans her moves more carefully, though eventually she gives in completely to her hunger which causes her to become more impulsive and rely entirely on her power to hunt down her enemies so she can consume them. As a result her hunger makes her similar to Kid Buu, as both are evil beings who are ruled by their destructive impulses which makes them dangerous to everyone around them, though 21's hunger makes her somewhat more predictable.
Super Warrior Arc
She is able to easily defeat Cell without even using half of her power. When inhibited by the waves she is able to easily defeat Nappa, the Ginyu Force and final form Frieza (who are all also inhibited by the waves). After absorbing Nappa, Cell, the Ginyu Force, and Frieza, she is defeated by the combined might of Super Saiyan Goku, Super Saiyan Vegeta, Ultimate Gohan, Piccolo, Krillin, Tien Shinhan, Yamcha, Super Saiyan 3 Gotenks and Good Buu. Vegeta states that if he had access to his full power, he would have been able to defeat 21 alone.
Enemy Warrior Arc
Even while inhibited by the waves her powering up in her base form is enough to shock Krillin. Even after having absorbed many clones, she is annihilated by the combined attack of Final Form Frieza, Super Saiyan Goku, and Perfect Cell when their full powers were reawakened by Bulma overriding the suppression waves.
Android 21 Arc
When inhibited by the waves, teamed up with Androids 17, 18, and 16, 21 is able to defeat Cell - even after he has drastically powered up by using the artificial soul he has been implanted with.
Android 16 describes her as having more potential than Cell ever had and being far stronger than him. He states she can endlessly power herself up by feeding on other creatures and that her urge to feed is her most dangerous quality.
Statements by guidebooks and authors
She can compete with the series' strongest fighters.
Red ribbon army
Appearance: Android 21 is described as a scientist with glasses and long bushy auburn hair. She has a gold ring on her left middle finger, and black nails. She wears a pair of hoop earrings, a sleeveless, blue and red short dress with black stockings, black arm sleeves, and blue and red heel boots. Her eyes are similar to that of Dr. Gero's Androids, such as Android 18.
Majin Android 21 in game
Android 21 in her True Form
In her true form, Android 21's skin turns pink, hair turns into a much more pale palette of pink, her ears pointed, her eyes changing depending on if her evil half takes over, and gains a tail. Her attire changes to a black tube top, keeping the black arm sleeves, and also gaining white baggy pants. It's noted that she most resembles a Majin in this form.
Her appearance in her true form is the result of her having all of the traits of the Z Fighters, Frieza, Cell and Majin Buu rolled into one.
Android 21 reveals in a private talk with the player that while she may have the appearance of an adult, she is actually less than ten years old, and regrets not having an adolescence, finding it awkward when small children call her "ma'am".