Warning: spoilers ahead for Blue Exorcist chapter 132!
Fans who have only watched the anime will understandably conclude that Blue Exorcist is about Rin Okumura rather than his brother Yukio. But the latest chapters of the manga prove otherwise.
What made Rin such an obvious hero in Blue Exorcist was how, as Satan's offspring, he wielded the demon king's blue flame and fury. This caused him to contend with conspicuous problems early on, including having to hide his identity from his fellow exorcists to avoid suspicion and, consequently, their fear. Once he was discovered, the prospect of his inevitable execution always hung over his head. Then there was the whole conundrum involving his broken Koma sword that originally locked away his true inner demon, though he seems to have easily solved this unfortunate dilemma rather quickly.
Throughout the majority of this, Blue Exorcist portrayed Yukio as nothing more than the sterner, more experienced and surprisingly younger brother who'd ostensibly been spared the same burdens that had so unluckily befallen Rin. But this couldn't be farther from the truth. Even the anime teased at Yukio's importance during his confrontation against the Demon Eater Saburota Todo, but fans who rely on anime have yet to learn more, as season 3 still has yet to be announced. As early as chapter 42 in the manga, Blue Exorcist began hinting at there being something wrong with Yukio, going so far as admitting he feels dead inside and wondering if something is eating away at him. As other characters begin noticing a troubling change in Yukio, chapter 69 reveals that one of Yukio's eyes sometimes releases the same blue flame as his brother Rin. Yukio becomes so obsessed about learning the truth and becoming stronger than his brother that he goes to extreme measures to unlock his eye's power by putting himself in mortal danger.
Yukio's descent escalates in chapters 76 through 77 when he attempts to make a morally questionable contract with the demon Hatshiro, ostensibly to save Kirigakure Shura and surpass his brother Rin. Although this only proves to be a diversion so Rin can defeat the demon, a later conversation gives this moment a darker twist when Shima suggests that lies are just another version of the truth - to which Yukio doesn't object. After more unsettling moments in chapters 90 and 91, Yukio ostensibly betrays his brother and friends to join the evil Illuminati organization, shooting his brother in the head for the first of many times. Although Yukio was planning to betray the Illuminati and eventually does, Yukio still refuses to come back and fights Rin. What's especially tragic about the whole situation is that Yukio learns that he's actually not special like his brother as he'd hoped. Satan is just using his eye as a stepping stone to get to his real target, and refers to Yukio as nothing more than a plaything for Rin.
Although Rin and Yukio finally reconcile during their multi-chapter battle, Yukio is eventually sidelined when Rin faces off against Satan in the final war, which will be concluded later this year after a nearly yearlong hiatus. Rin's overall journey and struggles throughout are much more straightforward and less tragic than Yukio's - they are even forgotten at times. Readers are aware of Rin's plight, which is marginalized by how the worst-case scenario - his execution - keeps getting delayed, trivializing it in the process.
Meanwhile, Blue Exorcist shrouds Yukio's burden and fears in mystery for a majority of the series, forcing readers to feel more drawn to whatever is haunting him. Even though Rin possesses Satan's blue flame and is capable of unleashing a great deal of destruction, the darkness that surrounds Yukio is much more menacing, especially after his questionable actions, which are spread out equally throughout the series as opposed to Rin's story that's explored in short, sporadic bursts. The irony is that Yukio, who's always wanted to prove that he's just as strong as Rin, eventually realizes that he can't be the hero and must settle with praise from his brother. Yukio isn't just the main protagonist, but the tragic hero of Blue Exorcist.