The Soul Society is home to the departed. The majority of these souls are not shinigami, nor do they have the potential to be. They live their afterlife until it is time to be reborn as a human once again. Those souls that are not shinigami dwell in Rukongai, leading lives similar to ones they might lead in the human world and, despite being souls, are able to have children. Hidden away in the the province of Junrinan, in the First District of West Rukongai, there was a tiny boy with enormous potential. They say that those who start from humble beginnings have the greatest rate of growth, and this is most definitely true in the case of Hitsugaya Toushirou.
❝I hear a voice. It echoes. It crushes. It envelops.
It rumbles in the distance like thunder.❞
As a (younger) child, Hitsugaya is plagued with recurring dreams. In these dreams, he is in a barren field of ice, and he hears a voice far in the distance. He can't understand the voice, as it is too far away, but he feels that the ice has a "presence." Despite the voice being intangible, he still feels it powerfully, like the charge of electricity in the air from a distant lightning storm. He finds these dreams perplexing, but he seems to consider them simple dreams and nothing more.
Hitsugaya lives a humble life with his grandmother, who may or may not be truly related to him, as it is unspecified if he was born in the Soul Society or is a soul that came to it through death. Housing in the Soul Society is based upon time of death rather than family, so many who live together become family by association rather than blood. This is likely the case with Hitsugaya and Hinamori, who grew up together as childhood friend. The list of people important to a young Hitsugaya abruptly ends there, however.
❝Everybody says I'm like ice.❞
Hitsugaya is ousted from his community, by children and adults alike. He doesn't seem to understand why, and questions whether it is his appearance or his personality. He is described by those around him as cold, and it appears to have affected him from a young age. Constantly being excluded has left its mark, forcing him to harden his heart and live on guard. This makes it easier for him to dismiss whenever people avoid him, and while he takes note of it each time, he doesn't let it bother him as much as it could. But though he is strong in the face of isolation, he still longs for a place to belong.
His status as an outsider has made him incredibly appreciative of those who don't treat him as one. Though he is very guarded, he speaks and smiles genuinely in the presence of his beloved grandmother. His precocious attitude is something that he developed very early, but it is lighthearted with his grandmother. His dynamic with Hinamori has far more banter, but it too is lighthearted. He places tremendous value in these two important figures, and seems to feel that as long as those two love him, it doesn't matter what the rest of the community thinks or says. He never seems apprehensive about going off on his own or running errands, even if the businesses he buys from are curt with him.
Eventually, Hinamori enrolls in the shinigami academy. He acts as if it is not a big deal and sends her away, continuing to live quietly with his grandmother. He seems to always act strong for them, keeping his real feelings hidden in order to not worry his grandmother and not hold Hinamori back.
❝(Hinamori) doesn't spend much time here anymore.❞
Five years pass since she enrolls, and he notices that her hair has gotten longer and she seems to have a goal. He then states that he hasn't grown an inch. We can guess that he probably feels alone and stuck, watching Hinamori move on and find something she is passionate about and, better yet, something to strive for. His statement about himself not growing likely has little to do with his height and more to do with his spirit. For him, nothing has changed, and he doesn't know how to change it. He doesn't seem likely to try either, as he focuses on how thin his grandmother has grown in the past five years. He is concerned about her health, and opts to keep conversations and situations light, likely to keep her from worrying and possibly getting sicker. When she seems as though she wants to speak seriously with him, he immediately volunteers to run to the market and buy sweet buns. Honestly, he is probably afraid of having that talk with her, afraid that the last person who loves him will leave him too.
When he goes to the market, it is business as usual with the shopkeeper. He makes his purchase and reaches his hand out for the change, but the owner wordlessly drops the coins on the counter to avoid addicentally touching him. He seems mildly annoyed but unfaed, until he hears a loud voice and is literally almost suffocated in a female shinigami's chest. He is completely bowled over and collides with the ground while the woman, who is Matsumoto Rangiku as yet unfamiliar to him, loudly berates the shopkeeper for the way he treats his customers. She then grabs Toushirou from the ground.
❝Don't let that guy push you around!❞
He is angry because the shopkeeper isn't the one that pushed him around - it was actually her. She pauses as if something about him struck her as peculiar and she's trying to figure out what. He takes the opportunity to make his escape as she hesitates. She calls after him, but he ignores her and goes home.
Later that night, his strange dreams return. Only this time, the source of the voice is visible, and it's this massive awesome serpentine dragon that towers over him. Hitsugaya is (understandably) shocked by the sudden appearance of this creature, and it tries to talk to him. He catches bits and pieces of what the dragon is trying to say, but the wind picks up and drowns out its words.
❝Boy! Are you my... My name is...❞
Hitsugaya seems desperate to find out what the dragon is trying to tell him, and is noticably distraught that he still can't hear as he shouts through the wind. He seems to have this innate sense that whatever those words are, they are incredibly important. Hitsugaya's need to understand is just as urgent as the dragon's need to communcate with him. As if he knows, even if only subconsciously, that whatever those words are, they will completely change his life.
Hitsugaya is startled from his slumber and wakes up face-to-face with none other than the female shinigami from the marketplace. He's about to ask what the hell she's doing in his house when her cheer fades and she tells him something completely shocking.
❝You're freezing your grandmother to death.❞
All the times the townspeople referred to him as "cold," they meant it literally. Hitsugaya has incredibly strong spiritual power that he was completely unaware of, powers that are so trong that they physically manifested around him, lowering the temperature of anyone in his vicinity. The community was scared of him because of this unrelenting, uncontrolled reiatsu that was pouring out of him like water out of a cup that was already full. He never even realized because this power is a part of him, an extension of himself just as natural as a limb. Because he was the source, he was never affected by it. It was never his appearance or personality they feared, but his sheer power that they instinctually avoided.
In discovering this, he also learns the truth behind his grandmother's gradually worsening health. Every night, she was fighting for warmth while he slept beside her. That nightly battle took her strength, little by little. Matsumoto explains to Hitsugaya that staying with his grandmother will be thething that kills her. She asks if he hears a voice, but she already knows the answer. She explains to him that the only way for him to learn to control his power is to become a shinigami like her. Ultimately, Hitsugaya decides to leave home and enroll in the shinigami academy.
❝Seeking a place to belong, I decided to go forward...❞
His grandmother gives him her blessing. She confesses that she felt as if he was holding himself back for her sake, and that there was nothing that hurt her more than watching her grandson's suffering. I'm not sure if she knew the reason behind her illness but chose to stay with him regardless, as she loves him dearly and the effects of his reiatsu aren't his fault, or if she simply thought she was deteriorating due to age. When she tried to talk to him before he went to the marketplace, she could have been trying to warn him that she might not last longer or tell him not to hold himself back out of concern for her, or maybe she even was trying to tell him that she thought he might have the makings of a shinigami. No matter what she wanted to say, she is happy to support her grandson in his decision.
Part of the reason Hitsugaya leaves is to keep his grandmother out of danger. But I think another part of him wants to leave that horrible community that completely ousted him. The only place that ever felt welcoming to him was his own home, and though he would never feel resentment towards his grandmother or Hinamori, they were the only things that kept him attached to Junrinan. Leaving it behind will help him discover who he truly is, what kind of hidden strength he possesses, and will help him find a place where he belongs.
The shinigami academy was originally just a way to find out his own goals and place. He discovers that the dragon in his dreams is the manifestation of Hyourinmaru, his zanpakutou. He is surrounded by shinigami who aren't afraid of him. He immerses himself in studying and training himself to use his power. And he discovers that being a shinigami is his calling and that Seireitei is the one place that he can truly call home.