It's not because Tadashi ever genuinely disliked Hiro. He just didn't really like him for a long time.
It was somewhere between three and four months after Tadashi's fifth birthday that his parents told him that a baby was growing inside his mother's belly. Which Tadashi got excited over because of how excited his parents seemed. Because Tadashi was a small child and thought of his parents as almighty and omniscient, so if they seemed excited over it, it was something to be excited over. But like a lot of children, even the abnormally smart ones, he soon enough forgot about it and only remembered again months later when his mother got so big that one of his classmates asked if she swallowed a watermelon seed or if she was just starting to get really fat.
It was also around that time that his parents started to buy the bigger stuff for the new baby. This mother has been buying here and there, even before she was pregnant, but the only things the they had at the moment was a few toys, the clothes Tadashi had outgrown, and an old crib his mother said possibly was full of mould after having been put away for so long.
It was really exciting for Tadashi because his parents always smiled so much when it was about the baby and always listened to and sometimes even asked for Tadashi's options. The only thing he didn't like was when they laughed over stuff he didn't understand. They didn't seem to laugh mockingly, but it still sometimes felt like they laughed at him. Like when he had asked one day if the baby would want to play with him when it decided to leave his mother. His parents didn't laugh then but they did look at him in a weird way, even if their eyes were loving, and explained to him it would need to grow some more before it would be able to do that even after it was born. To which Tadashi asked if it needed to be watered like a plant. And his parents laughed. They laughed so hard his mother got tears in her eyes and Tadashi hated it even if he normally loved it when his parents were happy. He just didn't understand why they laughed at him, so for a while he refused to talk or ask about the weird thing in his mother. It would have been for a long while if he didn't find two identical seashells and since there were so much water around the baby, at least that's what he was told once, he would be able to hear his sibling if he put one of the seashells to his ear and the other one to his mother's stomach. When he told them they got the weird but loving look in their eyes and told him it was a great idea. They even asked if they could hear too, which Tadashi of course answered yes to.
Months passed and one day his aunt Cass picked him up from school. He asked her "Where is otōsan and okāsan?" To which she answered, "The hospital, sweetie" It made Tadashi cry until Cass frantically explained to him that that was where people went to deliver babies, and after that told Tadashi that yes it was stupid to make it sound like the baby was a package by saying it was delivered, but it really was a common expression.
Tadashi arrived home and his aunt tried to help him with his homework since he already was already in 1st grade, but she soon realized she did more harm than good and let Tadashi be until he was done. Which was good because Tadashi was distracted enough by the thoughts of his new sibling. After a far longer time than Tadashi normally did his homework in he was done. He was about to ask Cass if she wanted to build something with him, since his dad said he wasn't allowed to do it without an adult, but he heard a car pull into the driveway and then looked out the window and saw it was his parents.
More excited than any of him or the adults around him thought he would be, he ran out to his parents. His dad threw him up in the air beaming with happiness. Tadashi was soon on the ground again and ran to his mother who took a big baby seat out that contained a small, reddish baby. His mother smiled big at him and said, "See Tadashi it's your new brother, your Otōto. Isn't he beautiful?" Tadashi's first thought was "No not at all", but since Tadashi was a smart child he knew it would upset his mother so he just smiled at her. His mother then managed to smile even bigger than before, which Tadashi didn't think was possible and he hoped she couldn't smile any bigger because it was starting to get creepy, and with that big smile she told him, "If you're done with your homework, you can show your new brother your robots. I'm sure he can't wait to hear what his nii-san has to say" Tadashi looked at the sleeping baby and seriously doubted it but he had already planned long ago what he wanted to show what he now knew was his brother, so he just agreed and walked with his mom into his bedroom to show his brother all the cool stuff he had. His mother put the seat on the ground and left the room to let her sons have some privacy which Tadashi appreciated, even though she left the door open and Tadashi could hear her walking just outside his bedroom.
Tadashi took out his robots and saw his brother was awake and looked at him unfocused. For a moment he just stood there looking at his brother who looked back at him, but then showed his robots and what they could do. At first he felt silly for doing it but before he knew it, he had shown almost all his stuff. He even showed his school books and blue lava lamp. He only stopped because his parents came into the room and said it was time for dinner and took his brother and put him in a mouldless crib before Tadashi and his parents had some dinner.
Tadashi thought almost everything about his baby brother was cool for about a week and a half. He learned that his brother was named Hiro, which was a cool name, Hiro never interrupted anything Tadashi said, and Hiro and his parents had walkie-talkies which was extra cool even though his parents called them baby monitors. Tadashi also got to be what he called Hiro's hero and find just the right diaper since his parents always just took the one on top and it was almost always the ones in the bottom or middle that was the right one. But after that week and a half Hiro started to become just as boring as he was when Tadashi first saw him. Sometimes he was even annoying with all the crying, throwing up, and the fact that Hiro literally shat himself many times a day was not only annoying but also gross. So Tadashi let Hiro be. Sure, he still talked to him from time to time, since he was the best listener Tadashi knew, and even held him a few times to please their parents, but most of the time he just ignored Hiro. It was not like he disliked Hiro, because he never actually did anything bad towards anyone and their parents didn't ignore Tadashi like some of his classmates told him they would when Hiro was just a few days old. Tadashi just didn't like Hiro. Even if Hiro was what adults called an easy baby and was almost was completely quiet when Tadashi turned 6 just about a month after Hiro was born, he was still not able to keep Tadashi's interest.
Years went by and Tadashi learned to play with his brother but also that 2 year olds could be extremely annoying. Tadashi still more or less just let Hiro do what he found amusement in unless it was taking the 8-year-old Tadashi's stuff and drawing or otherwise ruining it, again. Their parents told him that Hiro was extremely smart but Tadashi couldn't see it at that point. Maybe it was because of how little Tadashi actually acknowledged Hiro's presence or that when Tadashi last looked at him that day Hiro was chewing a candle and after that tried to do the same with a lit one. At least Hiro now looked less like a baked potato and more like a normal child, so Tadashi supposed that Hiro was a tiny bit better now than before. After a bit of thinking Tadashi decided that the fact that Hiro looked at him with his big, brown eyes like Tadashi was both almighty and omniscient helped Tadashi a bit in liking Hiro.
A year later a nine-year-old Tadashi wished Hiro died just like their parents did. He didn't dislike Hiro but he still didn't like him, and Tadashi realised just how much he both liked and loved his parents. If he could choose, he would trade Hiro in a heartbeat for just one of his parents. His aunt Cass cried when he told her that. He was smart so he knew she most likely would do that, but he decided it was her own fault for insisting he talked about his feelings with her. It was also such bad timing they died. Hiro had just started to like going to the very expensive genius school he got accepted into because apparently he actually was smart, way smarter than Tadashi was at that age, and it was also just a few weeks before his 4th birthday and a few weeks and a month before Tadashi's 10th birthday. Tadashi thought everything sucked and looking at his brother he saw that Hiro thought the same. Not that Tadashi used more time looking at his last close family member, but the way Hiro cried when he first was told he would never see his parents again, when he was told he couldn't go to the school he liked, and when he at the funeral truely realised that death meant never seeing someone again was heart-breaking, so it didn't take much to figure out what Hiro felt.
Time passed with nothing but moving to his aunt's house and weeping over his parents. Tadashi ended up getting a big room, bigger than even his parents room were when they was alive, but he didn't like it one bit because he had to share it with his brother. His brother who Tadashi couldn't look at without seeing both their parents. Just like he couldn't look in a mirror without seeing the same. His brother also acted so weird all the time. Most of the time he was just as cheerful as before but then he would just either look into the air, Tadashi, or his own things and start to cry out of nowhere.
Tadashi started to ignore Hiro more than he ever had until his own birthday. Tadashi had been to school that day which he normally hated to do on his birthday, but this year he liked how all his friends gave him a feeling of familiarity by the way they wished him a happy birthday. None of them did it in the way to happy way Aunt Cass did or kept quiet around him like some of the other people he knew. They treated him like one of them, just like they always had, even though they all were older than him. He dreaded having to go home to Aunt Cass's sicking sweet smile and Hiro's either way too loud cheerfulness or empty staring. But since he was 10 there were no way to get out of it and after packing his bag as slowly he could he went to his aunt's house. He was instantly greeted by customers and his aunt, but went past them saying he had homework to do. They let him go just like that and Tadashi didn't know if he hoped they would have stopped him or not. Maybe deep inside he hoped his mother would come and playfully look strict while saying there were no such thing as homework before he had had some birthday cake like she had done every year since Tadashi started in school.
It wasn't the cake that he missed though because he did get cake later that day, but was still glum. Well, he was glum until Hiro saw Tadashi had barely eaten one bite of his cake and asked if he wanted to switch plates since Tadashi's obviously must have some kind of flaw. Tadashi declined saying that their pieces of cake was near identical. For a while Hiro looked troubled, but Tadashi thought that was that since he didn't say anything for a while and therefore looked away again. He only got to look away for exactly 2 seconds before he heard a thud and turned his head and saw Hiro had smashed his piece of cake with his face. Both Tadashi and Cass was worried until Hiro lifted his head and smiled at Tadashi saying, "See, now they are different!" Tadashi was stunned for a while but then a laugh erupted from him. It started out small and awkward, but then grew until he could feel tears in his eyes. Soon enough he heard Aunt Cass did the same while Hiro just looked troubled once again.
Even after Cass had cleaned Hiro's face, after taking a picture of it and the piece of cake of course, Tadashi still couldn't look at Hiro. But now it was because all he saw when he looked at him was his face covered in cake and his stomach hurt from all the laughter. Hiro didn't entirely understand his brother's thoughts and feelings, but he understood that his brother no longer felt mad or sad when he looked at him, which made Hiro very happy. Hiro became even happier when Tadashi would show him some of the things he got for his birthday. In return, Hiro showed Tadashi a drawing of them flying on a big, white and red robot before Hiro handed it over to Tadashi as a birthday present.
It was a long process, but without Tadashi really noticing until it was done, he went from being pained by seeing his parents in Hiro to appreciate it and seeing all the wonders in Hiro and truly liking him. That being said he was still a child, and sometimes he would still cut Hiro off or hurt him even though he didn't intent to. Like when he saw Hiro surpass him in some areas despite being almost 6 years younger than Tadashi and he felt so jealous he couldn't properly understand it himself. Or when it was their parents' death anniversary and Hiro was playing as if it was any other day. Tadashi also felt jealous that day and that day he again couldn't look into Hiro's eyes and without painfully seeing his parents. But he, eventually, stopped entirely ignoring Hiro because even if he sometimes felt negative feelings because of Hiro, the positive ones always won.
One night shortly after having lived with his aunt for a year, Tadashi lay in his bed and caught himself thinking "Hiro is good enough" before he looked over at the other bed in his room. He then corrected himself, "No he is not good enough. Hiro is the best."