@0takuDragonSlayer
This kind of gets into "fanfiction" territory so forgive me if I inject my biases into these statements, but some steps I think that would help to develop Jiu-tae's character would be:
1.) Make Jiu-tae a more active character. So far events happen to Jiu-tae, he doesn't initiate events. Most of the narrative drive comes from Maria, which strengthens her character, but it adds to your point about Jiu-tae being bare bones and boring. He doesn't initiate any narrative action. When his friend, Ok-dong, is getting beat up, Jiu-tae only learns about it by chance before he takes action. There's multiple ways to rewrite this event so that Jiu-tae is an active participant here. For example, while eating/hanging out with Ji-eun and Ok-dong, Jiu-tae notices that Ok-dong is acting strange, or has a bruise, anything really. How does Jiu-tae respond here? Does he ignore it? Does he press Ok-dong? Does he "tail" Ok-dong to see what's going on? All three (and there are way more potential options here) of these choices would reveal something about Jiu-tae's character, it would show how strongly he cares (or doesn't) for his friends, and by extension, his family and we would learn how important it is for him to find his sister. If he cares this much about his friends (assuming he doesn't ignore Ok-dong's suspiciousness), then by extension he cares for his sister way more.
2.) Actually showcase how far Jiu-tae is from his sister by demonstrating how far down the Fight Class rung he actually is. As is, its really hard to get a grasp of how good Jiu-tae is at MMA for two reasons. One he's had about three serious fights in 47 chapters, and two, because his very first opponent was one of the best 16 fighters in his entire year, Tae-young, and he took her to 3 rounds. Despite Jiu-tae only have a month of MMA experience, he's clearly special. I think the tournament structure could have been changed to showcase how much work Jiu-tae actually needs to do to reach his sister. For example, if in his first fight he fights Fighter D, and loses (it can be a stomp or w/e might not be important), and then in the next round Fighter D loses to Fighter C, and then Fighter C to Fighter B, and Fighter B to Fighter A (Shin Ha-min most likely) we learn more about Jiu-tae's current skill level. This setup immediately establishes rank (A > B > C > D > Jiu-tae) and visually shows that Jiu-tae's got alotta work to do. Currently, he fought one of the best fighters in Korea, took her to the "limit", and then conveniently got cracked ribs so he couldn't fight anymore which is just a really strange decision. This doesn't give us any visual basis for Jiu-tae's skill as is, and it prevents further development for Jiu-tae because when we're considering story depth, a fight scene isn't really a fight scene, its a clash of characters, in the ideological sense.
3.) Use the fights to develop Jiu-tae's character. Test Jiu-tae through the battles, "how badly do you want to find your sister?" Show us the answer to this question with conflict. MMA is brutal, and it can be used to showcase Jiu-tae's will if, during the fights, he's questioning himself as to why he's going through all this pain. We see this in
chapter 8 when Kim Dong-won knocks him out, but 8 chapters later we get this
scene, where he takes a step back. Through the fights Jiu-tae's character can be developed, like against the guy who bullied him, Jang Chun-soo. I get why the author had Shin Ha-min beat him, but it would have been better for Jiu-tae's character if he fought Jang Chun-soo, and Jiu-tae had to confront his own weaknesses in that situation, regardless of the outcome. Would help reveal his character.
4.) Have Jiu-tae interact more with his supporting character and introduce more supporting characters. Within the story, the only characters Jiu-tae has consistently interacted with are Maria, Ji-eun, Ok-dong, Sa-rang (though we haven't really seen them have much of a conversation), and only his interactions with Maria significantly develop his character. A pretty good tool in creative writing is character maps. The basic idea of a character map is that, before you write the story, you try to map out all the characters in the story and you relate them to the main character, and use these relations to reveal aspects of the main character's personality, and issues. I've actually written a series of essays about this subject with Ballroom e Youkoso on Reddit, but for the sake of brevity I'll just say, in that manga all of the characters introduced relate to some aspect of the main character's goals and highlight his personality issues, and key characters constantly challenge the main character to do better. For Jiu-tae, he needs more impactful interactions with other character. For example, Ji-eun could constantly question why Jiu-tae is in the fight classes, which can cause him to question the point of what he's doing, and if his sister is worth it. There could even be a scene where he gets crushed in a fight, and starts heavily second guessing everything with Ji-eun supporting that decision until finally recommitting to finding his sister. Jiu-tae doesn't have much of this going on for him, and it adds to your point about him being boring. Another thing that's weird for me is that, in a class of 16,17 (?) people, Jiu-tae has had minimal interactions with any of his classmates on a character level. I can see the purpose here, to show that he's an outsider, but it feels off to me, but this might just be an off shoot of his lack of significant character interactions outside of his classmates. If his interactions with Maria, Ji-eun, and Ok-dong, were stronger, its possible this wouldn't feel weird to me, but idk bro.
These are just some ideas that would help develop Jiue-tae as a character.
EDIT: Yea, I with the Hajime example I meant to showcase the idea that you can show character growth in defeat, I didn't mean to insinuate that Ippo constantly loses. Because fights are actually story events, its important for the author to use those fights to showcase character development, and even in defeat a lot can be learned about a character.
