Yuma, Osamu, and Replica
Looking back at the first couple chapters of WT, I really appreciate the interactions between Yuma and Osamu. When Osamu gets bullied he doesn't fight back and chooses to ignore it, which Yuma finds baffling and not what he was taught growing up which is to always fight back with his own strength. If he's not strong enough, then retreat, get an advantage in numbers, etc. was what his dad taught him as a soldier in a war torn country. But Osamu, on top of being super weak, chose to fight for Yuma when he gets bullied without caring of his own well-being; even when a gate shows up he protects the bullies who were just beating him up from harm against the trion soldier.
Yuma then asked Replica if he should help Osamu that is struggling against the Bamster to which Replica, like always, says only he can make the decision himself. Yuma was indecisive while Osamu acts immediately without thinking; Yuma cared mostly about himself while Osamu disproportionately cares about others more than himself; Yuma's strong while Osamu is weak. The two are polar opposites so Yuma is baffled that due to his side effect, he can tell that Osamu never lied, not even to himself all this time and truly believes in his own morals. I think this is why Yuma follows Osamu, because he has never met someone who is his polar opposite and he wants to understand him.
Fast forward to chapter 76, in a fight against an overwhelmingly more powerful opponent being Viza, the old Yuma would've ran away, fought only to weaken the opponent for his allies to finish the job, or not even take the fight in the first place. But dude decided, against his better judgement and any input from Replica, that he was going to fight for the sake of Osamu, and fought with his life on the line just like how Osamu fought against trion soldiers with the odds stacked against him twice. I like to think Replica knew all along that they might be separated from Yuma in the future, so their catchphrase 'I cannot make that decision, only you can' was to prepare the indecisive Yuma that only fights for himself for that day. "Osamu, this is goodbye. Take care of Yuma."