Zero Two (Level 3) mail warning

Since childhood, Zero Two had been alone due to her Klaxosaur blood, always being seen as a "monster". The way people treated her as a child made her lose faith in people which is especially seen when she was a child: being very defensive or aggressive when people tried to interact with her. Besides that, Zero Two acted like a wild animal or a feral child, eating with her hands, growling, making other vocal noises instead of speaking and chasing after mice that were near her. However, Zero Two was also shown to a fairly cheerful, curious, and happy girl, and it was during her time with Hiro that shaped much of her personality and quirks in the present, such as her sweet tooth, the way she referred to herself as ("Boku"), and the nickname she had for Hiro, "Darling". Because Hiro treated her as he would anyone else, whereas others didn't treat her as a human for one reason or another, Zero Two developed a deep admiration and liking towards him. Despite having her memories of the darling from her childhood erased, Zero Two retained some fragmented memories which was why she strived to become human no matter what.

As she grew up, Zero Two's personality was rumored to be dangerous, cold, and alluring. She was an elite pilot with the pseudonym "Partner Killer", given the mysterious cases of her co-pilots dying after only three sorties FRANXX rides. However, she appeared to be playful towards Hiro and sometimes showed humor; which is evident during their first meeting, in which she teased him and called him a "pervert". However, she prioritized her duty as a Parasite more than anything, not because she wanted to protect Papa and the others, but because she wanted to "become human". Her outlook on life was quite cynical which is why she didn't bother with a name since they were all meant to fight and die. She had also been a prized military asset of APE and sent to fight on the front lines for so long that she had no regard for human life or her own since they all would become a statistic when they died anyway. When people around her died, she hardly batted an eye at it because, to her, it was just a fact of life that everyone dies. Zero Two always kept people at arm's length with mocking humor, nihilism, and emotional detachment. Zero Two also freely talked about how much she hated their society. However, after seeing Hiro's determination to pilot with her, Zero Two noted that they were very similar and she invited him to partner up with her, despite having fought alone during the majority of her life and never leaning on others for support.

It is later seen that Zero Two was a very unruly, passionate, fierce-tempered, self-confident, smug, assertive, and carefree girl who hated to follow the rules set up by the adults. She was very outspoken and honest, and unlike the other Parasites, she did not care about Papa and held no respect for him and the other Sages, only seeing them as people who lied to her throughout her childhood. Her seemingly care-free rule-breaking helped break Hiro out of his crippling depression and blind adherence to rules, and the cheerful affection she showed him inspired some of the other Squad 13 Parasites to be more affectionate with each other. Zero Two's behavior was also quite childlike, as evidenced by when she wiped her hands on Zorome's uniform. Zero Two also demonstrated impressive agility, performing somersaults and flips then landing on her feet every time. During the Girls vs Boy feud, Zero Two tricked the girls into believing she would cooperate with them, after which she tricked the boys into walking in on the girls while they were bathing, then proceeding to steal both the boys' and the girls' clothes. While being chased by Hiro, she teased him and laughed while running throughout the house. By doing that, she felt just a little more like a human.[3] While an exceptionally gifted parasite, Zero Two also had extremely odd quirks: being a sloppy eater who uses her hands, often licking people, such as herself, though she didn't like how she tasted; Hiro, who tastes like danger; Ichigo, who tastes sweet; and Ikuno, who tastes like a secret. She also often mused about how she wanted to go in the ocean, dousing her food in honey, and having no sense of personal space.

It was also implied that she knew more about the outside world than the other members of Squad 13. She also appeared to be more knowledgeable about different things than the others, as she knew what a "kiss" was, what came "after kissing" and the meaning behind "love". She also urged the Parasites to speak up for themselves and do what they think is right before they regret it and before "it's too late".[4][3] Zero Two also clearly didn't mind whether she's wearing clothes or not, judging how she didn't even bat an eye about Hiro seeing her naked and instead picked on him for holding her panties.

Zero Two also seemed to only care about herself, so much so that even when Hiro was dying, it seemed like she couldn't be brought to care when Ichigo demanded she takes it easy on him. That having been said, she had deep insecurities about not being human. Though she hates it when other people bring up the fact she's not human and when people call her a monster, she can be very self-deprecating about it herself and even calls herself one. Additionally, her hatred for Klaxosaurs stems from the fear of becoming one herself. Her desire to avoid this, however, caused her to act more monstrous. This problem escalated when she developed a hatred towards mirrors, as she was seen crushing the hand mirror that Hiro had gifted her in frustration, and proceeded to smash every single mirror throughout the boarding house, possibly due to them reflecting her more monstrous appearance.[5] It only gets worse later on, where she came to consider herself a monster not for what she was, but for the horrible things she had done. Zero Two did also not appreciate "friendship" and never took it seriously because she was convinced that Squad 13 would never accept her, so she didn't even bother trying to be nice to them or show consideration for their lives. Zero Two didn't display any concern for the lives of any pilot besides Hiro, because she thought of them the same she thinks of herself: they are all meant to live and die fighting Klaxosaurs. However, it is clearer in the following episodes that she really had come to care about the other Parasites in Squad 13 and felt guilty for betraying their trust but stubbornly willed herself to keep displaying no empathy for them and to outwardly insist that she never considered them true friends, just temporary allies.

However, after she realized that Hiro was indeed her darling from her childhood, Zero Two felt extremely apologetic and distraught about her actions. With Ichigo fed up with Zero Two's antics, however, she forcefully disallowed her and Hiro to even visit. Ichigo even proposed to Nana that unless Zero Two were to be kicked out from Squad 13, they would not cooperate at the upcoming mission, much to Zero Two’s disbelief and frustration. Later, Goro was finally able to convince Ichigo to let Zero Two visit her darling. However, when the squad arrived at Hiro's hospital room, he was not there. Hiro’s sudden disappearance enraged Zero Two; she felt deceived and proceeded to beat up the members of Squad 13, mainly Ichigo. When Hiro came back and unintentionally called Zero Two "a monster", Zero Two finally broke down, leading to her deportation.[6] Later when Hiro finally reunited with her, Zero Two was afraid of being touched and seen by him, because she considered herself a monster and didn't think she deserved forgiveness after what she had done. However, after Hiro fully forgave her, apologized for calling her a monster, and promised her that they would talk out their problems and always be together, they confessed their love for each other and became an official couple. After this incident and officially being accepted by Squad 13, Zero Two admitted that she finally understood what "being human" really meant; living together with other humans, helping each other while also being true to their feelings. Thanks to this discovery, Zero Two's personality reverted back to her former personality, becoming much more friendly, responsible, cheerful, and composed. She also accepted herself, as she could happily look at herself and her horns in the mirror.[7] Despite that, she was concerned when Hiro turned into part-Klaxosaur like herself, stating that she wanted him to remain human.[8] Zero Two was able to acknowledge her mistakes which is why she was determined to do everything right, especially in regards to her friends. She also felt guilty over her past actions and started having nightmares over the stamens she had consumed.[9]

Zero Two regained her curious and carefree nature, doing things in her own pace and stating that she was interested in traveling the world like the Princess in her book.[10] She also retained her playful personality as she was visibly seen to be sassy and energetic, not afraid to get what she wanted to without putting up a friendly fight. She began to exhibit a fascination with the extent of a human’s capabilities, as she remarked humans were wonderful at being able to leave a mark for the future via reproduction, something her body was incapable of doing. Furthermore, she became more open to her fellow comrades in Squad 13, openly socializing with them and enjoying their company, as she teaches them how to fish, encourages them to learn to cook for themselves, and helps plan Kokoro and Mitsuru’s wedding. She became very protective of Squad 13 and encouraged them to explore their true feelings and taught them concepts only available to the outside world. She also felt extreme anger towards those who interfere with the squad’s personal lives, as she was visibly dismayed at Papa and the others for destroying Squad 13's brief period of happiness and erased Kokoro and Mitsuru’s memories due to their decision to get married.