HEADCANON:
The catalyst to Shinjiro’s change
[ While Shinjiro has always been a reliable person even since a mere child, a certain event is what truly shaped him into the form of the stern, closed-off and emotionally introverted young man that we know him as - even before the events of October 4, 2007.
Though obviously not quite as close with Miki as her biological brother, during the four years of knowing each other before her passing, Shinjiro still grew quite close to the girl - even as far as to make her regard him as another big brother of hers, having known the boy ever since she could remember. After all, as Shinjiro and Akihiko were four years old at the time of their first meeting and Miki being slightly younger (I personally headcanon her based on fanart I’ve seen as something like a year or two younger at most) Shinjiro had been a part of her life almost as much as the to-be-boxer had. It was the same for the little boy as well, having looked after her and often done things to try to make her happy (like our beloved rank 10 doll story) and as the boys were her only friends, Miki became somewhat of a little sister to Shinjiro as well. He is someone naturally protective and caring, it was only natural for him to feel this way, as he did with Akihiko.
After the orphanage burned down taking Miki with its flames of destruction, everything about the boy’s naturally supportive caregiver personality was hugely reinforced - though to the point where its effects were quite negative. At this point the boy had become aware of Akihiko’s sensitive and emotional nature during the years spent together, it was obvious to younger Shinjiro that the tragedy would devastate him like no other event possibly could. Though traumatized and equally devastated himself, it was now up to Shinjiro to take over the protective and reliable role of an older brother like figure more than ever before.
Shinjiro has always been someone naturally selfless, putting the needs of others ahead of his own. Knowing the impact that losing Miki would have on his best friend, the older one of the boys decided to push his own grief to the side - a decision which transformed him into the solid, responsible and supportive person that he is, but also made him reluctant to show any of his weaknesses or inner struggles to most people around him and forced him to mature well before his time. He wasn’t even ten years old, but took it as his responsibility to stay strong for the sake of his friend, unwilling to reveal the impact that the tragedy had on him as to be able to offer security and support to the one person that needed it the most. So much so that Akihiko would become to mistake Shinjiro’s seemingly distant and silent reaction as simply not caring as much as he did, as Miki’s death not having the similar kind of effect on him as it had on Akihiko. “You should have felt the same, too!” he’d say, unaware of the internal struggle that Shinjiro went through to drown out his own feelings for the younger male’s sake.
He never got a chance to properly grieve Miki nor to show it in any way, unlike Akihiko. Forcing himself to suppress (ironically enough) such a painful memory and emotions obviously took its toll on him. Since it made him forced to grow emotionally quite prematurely, it lead to his (sometimes overly) serious attitude toward most things compared to what I headcanon was a more playful and humorous, definitely more naive of a boy than after the incident. Being forced to hold feelings inside in turn made him react to having his softer or more vulnerable side being discovered or acknowledged extremely defensively (the so-called “tsundere” behavior we all known and love him for) and awkwardly. Especially so with Akihiko. He, if anyone - despite knowing him better than anyone else - is the last person that Shinjiro wants to show his vulnerable side to. Which is why, for instance, he never went to him after the incident with Ken’s mother but instead relied on suppressants and ran away (the same thing Akihiko complained about upon discovering this; “why didn’t you rely on me instead?”) Just as was with Miki's accident, he simply couldn’t openly show his weaker emotional state to Akihiko. He was supposed to be the emotionally stronger one, dependable - the person to rely on, the one to look after him.
And Shinjiro never let Akihiko know about this. He never complained or thought he should be grateful that he was willing to sacrifice his own emotional well-being for the younger boy’s sake. Instead he kept quiet, let Akihiko misunderstand things without ever protesting. That is how selfless Shinjiro is.
This all becomes the most evident in the manga. While Akihiko once again complains about Shinjiro not having felt the same pain over Miki as he had, Shinjiro’s inner monologue states this (misunderstanding) being because he is “not like“ Akihiko. Because Akihiko “never changed” but instead remained the same “laughably innocent fool of a kid” as always. Implying to me that the orphanage fire that took Miki with it was indeed the very catalyst, the very event that shaped Shinjiro’s personality into what it was, which was later negatively reinforced by the events of October 4th even further. ]