SIT DOWN I'm gonna talk about how Osamu and Yuma are desgined to be polar opposites and how they end up complimenting each other
Let's start with the most obvious one:
Osamu is a human while Yuma is a Neighbor. They were raised in different enviroments; one in a single big planet where peace is the norm, the other in a harsh world consisting of multiple planet-nations where constant war wages. Yuma grew up traveling all around the Neighborhood, Osamu probably hasn't even gone abroad yet.
Now, their appearances. ABSOLUTE OPPOSITES.
Osamu has black hair while Yuma has white hair. Osamu has green eyes while Yuma has red eyes, which are placed on the OPPOSITE sections of the color wheel!!
Osamu's also tall compared to Yuma. Yuma is actually short compared to pretty much everyone other than Chika and Yotaro, but still. Short.
Also the C-Rank trainee uniforms they wear. Osamu wears the classic, white-orange male trainee uniform. Yuma on the other hand wears the old black-blue agent uniform. GUESS WHERE ORANGE AND BLUE ARE PLACED IN THE WHEEL
And extra detail but Osamu's eyesight is bad (glasses) while Yuma's eyes are fine
Trion, battle experience and fighting styles
Before diving into the longer text, let's point out the obvious again. Yuma is an Attacker, Osamu is a Shooter. Melee and ranged, mobile and stationary.
Needless to say, Yuma has YEARS of battle experience. He's practically a war veteran at this point. He can take on both trion soldiers and regular trigger users easily. As a cherry on top, he has a black trigger with quite the broken ability. His trion amount digit is 7 which places him to the high points of the "average amount" scale.
Being the first ace of Tamakoma-2, he's expected to be active in the front lines of Rank Wars. He relies on speed and agility when fighting, making him an unpredictable opponent that can pop out of anywhere if you're not careful enough. Grasshopper and Scorpion compliments his style this way (Spider too but we'll get to that later)
Osamu on the other hand has been a normal trigger user for less than a year, and in most of that time he had been trying to figure out what the hell to do since he is neither fast nor strong (both physically and trion-wise (a mere 2!!)). Most of his progression happened during the last four months, when he met Yuma and they both joined Tamakoma. Now he can defeat trion soldiers that come in a single batch, and even rock up a few human agent kills.
Contrary to Yuma, Osamu is more of a support unit. Ever since R5 he's been working from the sidelines, providing assistance when necessary. Out of the field, he's the strategist that comes up with the team's next tactics and gives commands according to those tactis once on the field. He mainly uses Raygust and Spider—a heavy blade and wires. This consequently makes his fighting style a much more slow and calculated one compared to Yuma's.
Their personalities
This is a trickier part for me as I suck at writing personality analyses, so I'll let Ashihara do the initial talking and go from there:
"Yūma and Osamu's difference is that Osamu is a child that endures quite a lot. Even if he's bullied, he thinks it's better to endure it. But Yūma is a child who came from somewhere completely different, he's a child who pays back anything done to him.
A character who thinks, if he doesn't pay them back for what's done to him, he'll be pushed all the way to the bottom. Because that's the kind of life he'd lead… so I think that is where the biggest gap between these characters is."
(Source: Question 6 from BBF, page 329)
Circling back to the very first point of this text; Osamu is from our world, Yuma is from the Neighborhood. Everything is different in these two places, from the culture to the enviroment, the laws and rules, and especially the people themselves. All these differences shaped up their personalities in the long run. Yuma had to toughen up and take on the people who disrespected him head-on, because if he didn’t, he'd be shunned and pushed to the bottom, labeled as someone "weak". Osamu doesn’t have this problem, as Earth is more "forgiving" when it comes to stuff like these. (big doubt honestly but it is said that Japan in World Trigger is more peaceful and lax than the Neighborhood so enh) Because of this, he thinks that it's the best to endure the bullying and the disrespect.
Speaking of, Osamu is incredibly oblivious at the disrespect thrown at him. That was proven when the Midorikawa chapter came up—"He doesn’t even realize what's going on," as Yuma said. I'm thinking it's got something to do with self-esteem or lack of confidence, which is another difference with Yuma who literally oozes with confidence as a veteran who fought on the battlefield for years. Quick to catch the shade and hostility that is thrown at any direction, most likely a skill he got from growing up in the Neighborhood. And when he catches on them, he starts "biting" the people involved—breaking the bully's foot, beating up people in some alleyway, that one rooftop stomp from chapter 3, the 8-2 Midorikawa match do I need to go on
Aside from the enviromental developments, there are also their other personality aspects. Yuma is more expressive and talkative. You leave him somewhere and he comes back with at least two new people befriended. If he doesn’t understand something he comes up and asks right away. Like I said before, he's confident about himself. He's generally someone who's easy to approach.
Osamu is more quiet and reserved. Usually keeps things to himself. He's also, as the lack of a better word, a nervous wreck. It seemed like he had little to no friends at school before Yuma came along and even now he doesn’t have too much of them in Border either (his connections don’t count that's literally just him networking). You can't really read him and tell what he's thinking about at a given moment because most of the time he has that nervous face, or if he's really focused, a blank face, plastered on. Has the guy ever smiled wide in the series?? The only one I remember is when Reiji told him to protect Chika with his life which... doesn’t really paint a nice picture 😬
Do you know about the different first person pronouns in Japanese that usually indicate how a person is like? There are a huge amount of them, but Osamu and Yuma use "boku" and "ore" respectively. "Boku" is a more humble pronoun that is used usually by men. It's in the middle of being casual and formal. It can also give a "mama's boy" or "teacher's pet" vibe to the person. "Ore" is casual, almost exclusively used by men. Depending on the person and tone it is spoken it can either give a "familiar, easy to approach person" vibe or a "rude man who shows off his masculinity" vibe. I'm sure you can tell which one of these refer to Yuma.
Going forward from a previous point, Osamu is someone that wants to unconditionally help everyone he comes across, not even caring his own life at times. Yuma calls him a "daredevil" because of it, as he tends to judge the situation, the people and his abilities first, take action second. As a result of this, Osamu's behavior remains alien to Yuma.
How they compliment each other
With Osamu's speciality at strategies and Yuma's ability to execute the more demanding portions of the plans, the two has always been a dangerous fighting duo. When Spider was added into the arsenal, the risk they posed increased tenfold because Yuma turned out to be incredibly skilled at hopping between the wires, making him an unpredictible opponent and Osamu evovled into a wandering support unit that places wires all around and hides, usually only showing himself when his assitance is needed.
Outside the battlefield, it feels like they have always been together from all the way back in chapter 1. Osamu tries to help his new, weird classmate adjust to Japan and it's enviroment while Yuma makes sure that his new, weird acquantiance doesn't die because of his idiocity. They learn more about each other during this time and start growing closer.
Chapter 21 is an important one for their relationship because of this. Yuma has seen enough of Osamu to reach to a conclusion about Osamu's personality by this point—an honest person that will throw himself at danger if he thinks it can help anyone in the slightest. He says later on that this behavior reminds him of his late father. Osamu on the other hand learns from Replica that Yuma is practically living on life support, that he doesn't have much time yet. (this is another difference if you squint. Yuma is actively dying while Osamu's health seems to be fine) He also finds out that Yuma, who now knows that he cannot reverse his father from the black trigger state, has no reason to live and will return to the Neighborhood to follow what can only be called a suicide mission. So he takes action.
They talk about Chika wanting to join Border to rescue his brother and childhood friend and Osamu asks Yuma if he wants to join them. When Yuma reminds him that he doesn't have anything else to do on Earth, Osamu pushes further. Then, Yuma asks that last question: "Why do you risk your life to help other people?"
I think it's Osamu's answer that finally convinces him to stay: "If I ran away from what I think I should do, even once, then I'd end up running away right when it matters the most. I know the kind of person I am."
I've seen a post on here saying that Yuma sees himself as a weapon, but he also thinks that he'd be a weapon used for worthy causes if he follows Osamu, since he's proven himself to be a good person, which is why he joins Osamu and Chika to form Tamakoma-2, and I think so too. (props to OP for writing that post btw it was an amazing read)
From that point onward, you barely see them not being around each other, or at least supporting each other one way or another. Osamu introduces Yuma to more and more of Japan's culture, language and rules while helping him with school and new friends and making sure he's enjoying himself. In turn, Yuma looks after Osamu's physical and mental wellbeing, does his best at Rank Wars using the plans his captain orchestrated and defends him from other initially hostile agents like Midorikawa and Hyuse (who said that he wouldn't follow Osamu's orders if he deemed them unworthy). He also says stuff like "I'd do whatever my captain told me to" and it's the sweetest thing ever
All these little exchanges built up mutual trust and loyalty between the two, which later turned into the ability to rely on each other in battle. It's like doing trust falls with your friends at school, one hundred percent sure that the person behind you will catch you no matter what. And this, in the long run, helps them compliment each other, because they both trust that the other knows what they're doing. So they make decisions based on the other's decisions, which makes it an even more complex plan and gets them the upper hand.
Thank u for coming to my Ted talk and I will see you all later in your notifications (because rest assured half of my daily routine consists of going through the world trigger tag)