Hiro often wondered who he was, as he says that the Garden was the entirety of his world and he always thought about it. He was in the Garden for as far back as he could remember. They lived there to become parasites who would fight for Papa and the others. All children were referred to by their code numbers. The lower the number, the higher the child’s potential as a parasite. The double-digits up to 099, the teens, in particular, were given special treatment. Hiro was one of them, and he was told he was special, unlike everyone else.
As a child, 016 gives himself the name Hiro when he tosses a ball, it rolls into Ichigo’s feet. She sits crying and doesn’t lookup. Ichigo wonders if she’s strange since everyone else has stopped showing emotion, though she continues to cry and says she wants to be like everyone else. She asks if she should get more injections. Hiro says that their codes are in the teens, so they were different from the others, to begin with. She says she doesn’t want that and being different is scary. And gives 015 the name Ichigo. Other children begin asking for names as well. Though they wait for their parents, they never return to them. Mitsuru shyly asks for a name too. Hiro smiles back at him. Two adults enter and say that they can’t change their designation without permission. Hiro politely asks what the problem is. The adults realize he’s one of the teens and simply tell him not to just do as he please before leaving. Hiro thinks that they never answered his questions and any other children ever asked any.
At some point, Hiro gives Ichigo a hair clip as a gift.
By the age of 9, Hiro became a special research subject and underwent tests at the Lab all by himself. If he did well, he was given a reward. One day, after testing, he witnesses Zero Two being dragged along for testing. Hiro asks an adult who “the girl with horns” is. She becomes frustrated and scolds him for always asking questions and orders him to his room. There, he decides to find out for himself who she is. After testing, he tries to sneak off but is stopped by an adult. He fails in his other attempts and runs out of ideas. Outside, he notices the girl trying to get her book back from an adult. He scales a large tree to look through her window. Zero Two manages to take the book back and the woman gives up. For a reason unknown to Hiro, he is overjoyed to see her happy.
As Mitsuru walks off with an adult Hiro asks if he’s going to the lab and if there is something wrong. Mitsuru says he’s going to get an Elixir Injection, as he won’t be able to stay if he doesn’t. Mitsuru says he wants to become a parasite with him. He asks if he comes back alive if he would pilot with him. Hiro happily agrees and the two make a pinkie promise. Afterward, Hiro, Goro, and Ichigo quietly walk back together. They find that Code 494 was taken away and another notes that it was Code 367 before that. The three looks on, scared until Hiro says they have to do something. Goro says that Papa and the others made the decision and Ichigo says there’s nothing they can do. An adult suddenly appears to collect Hiro for testing. Though Hiro asks where the other kids went, the adult ignores his question and tells him to get ready.
As Hiro undergoes testing, an adult notes that his sensitivity is troubling and may affect the other children. Dr. FRANXX observes him and a man asks if they should increase his yellow blood cell dose. Dr. FRANXX says no because he is interested in what kind of parasite a child like him will grow up to be. After finishing his testing and receiving candy, he leaves the room alone. He hears screaming and goes to investigate. He rushes over to find Zero Two undergoing painful testing with a group of adults surrounding her horrified he falls to the ground. A man suddenly appears and orders him to return to the Garden. He asks him to stop causing her pain. The man says it’s none of Hiro’s business and pulls him away. Frustrated with his situation and unwilling to watch the girl suffer anymore, Hiro packs a portable searchlight and scaled one of the trees near her cell. She had just gone through testing and is now curled up on the floor. Hiro shouts and slings his searchlight at the window. Shattered glass carpets the floor and the wind gushes into the room. Though Zero Two is initially aggressive, she sees his tears and begins to crawl towards his outstretched hand. When they both put their weight on the branch, it breaks and they fall to the ground. Hiro loses consciousness.
Then he’s carried through by Zero Two the snow until she drops him. Hiro wakes up and finds Zero Two eating a mouse and when he tries to get her to spit it out, she bites him. To get her to stop, he begins petting her hair. This reminds her of her “mother” and she lets go. He calls her mean and she appears to be ashamed. He realizes that she can’t understand him but introduces himself anyway. Though he asks her name, she can’t answer, and he sees on her ankle that her code is “002.” He thinks of a name, first thinking “Oni,” then “Zero Two.” He thinks they’re both bad, but she begins to try and say “Zero Two.” When she continues to say it, he says he should have thought of a better name but begins to call her that himself.
Zero Two notes that she didn’t understand his words, but knew his voice called to her, saw new things with her, and walked with her. He makes her shoes.
Hiro hands her one of his candies, but she doesn’t understand with it is. He tells her to go “Aah,” and slips it into her mouth. She enjoys it and Hiro eats his own, enjoying her reaction. She shows him her book, “The Beast and the Prince,” and he notes that the pictures are pretty. As he begins reading, a group of adults appears, looking for them. The two hide and Hiro tries to calm her down. One man asks what they should do with 016, and the other answers that 002 takes priority and to kill him if he resists. Hiro pulls her closer and promises to protect her. The two continue to flee from the adults and they eventually fall into the freezing water.
The two sat under a tree and Hiro finishes reading as Zero Two lies against him. He tells her that it’s sad they can’t get married. When she tries to say “married,” Hiro explains what marriage is. When he looks at her, she smiles back. He says that they should get going, but she falls when she tries to stand. She had scraped her knee and blue blood oozes from the cut. He looks for something to bandage it but is forced to lick it instead. He explains that animals lick their wounds to heal them. With him kneeling before her, she confuses his pose with the prince proposing. She asks if it’s marriage. Smiling, he says that they will once they escape. He tells her he wants to be with her forever too and become her darling. She begins crying and blushing and hugs him close. They look up and notice that they are under a mistletoe. They are suddenly interrupted by a group of adults. Though Hiro tries to protect her, he is hit with the butt of a gun and falls to the ground bleeding.
After that, their memories are erased. Hiro forgets his promise to Mitsuru and stops naming other children. Subsequent tests reveal that his aptitude results are drastically dropping. Despite this, he continued receiving special treatment from the adults.
Approximately five years later, at age 14, Hiro is discharged from the Garden and assigned to Plantation 13, where he is paired with Naomi. During their capacity test, Naomi and Hiro try to connect but their positive pulse and P-Factor drop and they time out, causing them they fail. As protocol, they are to return to the Garden, although Hiro, as a special specimen, is offered to stay. However, he declines out of despair not being promoted as a parasite.
Hiro has a standard “slim” male physique with black, shaggy hair and rich blue eyes. His checkered past has left him with an emphatic expression on his face, despite his personality. He wears the standard Stamen uniform.
As the story progresses, after he survives three rides with Zero Two, Hiro begins slowly develop his “Saurification” process in episode 13 where his canines start to grow. In Episode 16, he begins to grow small blue horns which become more visible until episode 23, where his eyes also change from dark blue-green into bright electric-blue with a blue eyeliner just like Zero Twos.
In Episode 24, His horns now become longer, like when Zero Two was a child, and he developed fangs. Shortly after their success to defeat VIRM, Hiro who later fully a human-klaxo hybrid have blue skins and gray scleras like most of the Klaxo-sapiens in a flashback from 001.
As a child, Hiro was very jovial, optimistic, and hopeful, giving names to other kids and to himself as well. He taught the kids that names only given to themselves could give a sense of uniqueness when the only name they had was their codes. This earned him fame among other children, even those in other plantations. Because of this, he was chosen to represent the Children in the Garden. During his time at the Garden, Hiro was very curious about himself and the world around him. He would constantly ask himself questions and whenever he tried to ask the adults, they would never give him an answer. This, coupled with the fact that the other Children never took a stand with him and were very obedient, made Hiro frustrated and he felt like he was the only person fighting the world all by himself. This changed when he met Zero Two, to which Hiro realized that there were other people fighting the world by themselves. He was then shown to be tenacious and brave, as he didn’t bother to wait to rescue Zero Two and broke her out of her room, and helped her evade the authorities for a while all by himself. However, after getting caught, the authorities decided to erase both of their memories: Hiro’s curiosity, free-spirited nature, and meeting with Zero Two were a few of the reasons why. After this, Hiro became less curious, joyful, and creative, and more emphatic, serious, and obedient. He also stopped naming children and was stripped off of some of the original aspects of his personality. Prior to his memory wipe, Hiro only wanted to pilot in as much as it would let him leave the Garden, but when this motivation was erased from his memory, piloting was all he remembered and his sole purpose for living.
Because he was considered a role model, all of the children hoped he would lead them because he was the best pilot, but Hiro’s aptitude skill began to dip after he ingested a young Zero Two’s blood. This inability to pilot a FRANXX had left his hopes low, even to the point of choosing to leave the Thirteenth Plantation until his former partner Naomi encouraged him to stay. Losing the ability to pilot had left him with extremely low self-confidence and left an insecure feeling that his life had little worth. He initially struggled to find somewhere to belong, even to the extent that he was more than willing to ride with Zero Two even if it meant death. Part of the reason why he was determined to keep on piloting Strelizia is that he was finally able to fulfill his role as a Parasite, and he was willing to die fighting than live as a useless civilian. However, this wish of his came to an end after he survived his third ride with Zero Two. It was then he realized that he had found another reason to fight besides protecting Papa and the adults: to become Zero Two’s wings.
Hiro appears to be quite philosophical, as he was first introduced as him monologuing about the philosophical meaning of a wounded bird he saw in the forest, indicating that he still had retained the curiosity of the outside world from his childhood. In his time in the Garden, he spent his time teaching Ichigo and Goro about many old but meaningful sayings that would stick to them even as they grew up.
Hiro was shown to be very hardworking and devoted, as he refused to stop piloting with Zero Two and when he trained in a FRANXX training unit all day and all night so he could be useful and fight alongside the other Parasites. He was also kind and caring, as despite feeling depressed over his inability to pilot, he never acted in a harsh manner towards his fellow pilots, even when some of them weren’t as welcoming towards him. However, he was more than willing to fight back when Zorome antagonized him several times. He was determined to prove himself worthy to pilot and make himself acknowledged. When he got promoted to an official Parasite, he states that he would do his best and not tie them down. After regaining his memories, Hiro regains his former personality. Also, thanks to Zero Two’s influence and his bonds with people, he understood that there was more to life than just to pilot a FRANXX. He also began thinking about the future and all the possible paths that were open for all of them. He teaches all of this to his friends in Squad 13. Instead of the “fight until I die” mentality, Hiro developed a “fight so I can keep living” mentality. His traits from childhood also started to resurface: he became fearless, disobedient, rebellious, and ready to stand up for his friends, even it meant going against the rules. This is seen where when Papa and the others destroyed Squad 13’s a happy and peaceful life, especially in regard to Mitsuru and Kokoro’s memories being erased due to their decision to get married, Hiro told Papa that he would never see him as their âpapa’ nor obey him ever again after what he did to his two friends, insisting that the Parasites would be free from APE after their final battle. He became increasingly adamant about gaining freedom for himself, Zero Two, and their teammates. Hiro also develops a strong dislike towards those who hurt his friends, and he told Dr. FRANXX he will never forgive him for the torturous treatment he put Zero Two through and felt disdain towards the 9’s for their interface of Squad 13’s personal lives and frequent mocking of them. Nevertheless, Hiro is willing to set aside his personal feelings in order to do what he believes to be the right cause, such as pairing with Alpha in order to rescue Zero Two.
Hiro was a person that could do anything if he put his mind to it. As the story went on though, Hiro would rarely let his friends affect the decisions he took by himself. He also had the tendency to keep things to himself even when sharing them would be beneficial for everyone, which then leads to serious cases of miscommunication. Even when he was struggling, he kept to himself because he did not want to trouble the others. This is the case where Hiro asked Goro to keep quiet about his tumor-like mutation since he was devoted to keeping piloting with Zero Two. As stated by Goro, he often jumps into things by himself and takes his own decisions without talking with his teammates first. Although Hiro does feel guilty for hurting his friends’ feelings and letting them down, he wants them to understand that he feels it is important to chase after and protect what he loves the most to the point he would rather die trying to protect it rather than do nothing and live a life full of guilt.
Ever since Hiro met Zero Two, he had been thinking about her, her personal life, and her feelings as he wanted them to understand each other. He also wanted to help her with whatever was weighing on her mind and get closer to her. Rather than his initial thoughts where he would rather be dead than cease to pilot, Hiro later admits that his resolution to live is to be with Zero Two, and if he can’t see her smile, he would rather be off dead. The two have shown unconditional love for one another and know that their bond can overcome whatever obstacle fate throws at them. Zero Two credits him for encouraging everyone to live their lives for the greater good but Hiro feels it is not something to be proud of because he believes he only caused too much trouble to his friends, but she persuades him that it was because of his desire to live. Right before his demise, Hiro and Zero Two were completely willing to sacrifice themselves to save the Earth and so their friends and the other people could live a good life. They state that even if they would die, the paths they had walked would never fade since their friends would take them even further.