WARNING: The following contains spoilers for Season 5 of My Hero Academia, now streaming on Crunchyroll, Funimation and Hulu.
When Deku first inherited One For All, it was important that he kept any and all information about his new Quirk tightly under wraps. However, deceit isn’t one of the My Hero Academia protagonist's defining traits. He ended up revealing the secret to his friend and rival, Katsuki Bakugo.
Bakugo got the first hint of One For All early in their U.A. days when was in a sour mood after Deku had beaten him in a training exercise. Thinking it would make him feel better about the match, Deku told him about One For All. It didn’t seem like the greatest idea at first, but Bakugo’s knowledge of One For All is proving to be quite helpful for both Deku and All Might in Season 5.
Near the start of My Hero Academia, Deku didn’t go into any specifics about the Quirk, only saying that it was someone else’s power he’d been given. He also promised to make the Quirk his own one day and defeat Bakugo with it. Naturally, the story about transferring power seemed ludicrous to Bakugo, who dismissed it as a poor attempt to make fun of him.
Bakugo didn’t give Deku’s confession any more thought until after the Kamino incident when he found out about All For One’s existence. Suddenly the idea of receiving a Quirk from someone else didn’t seem so far-fetched, while Deku and All Might’s behavior fueled his suspicions further. He deduced that Deku’s power had somehow come from All Might, and confronted him with this information.
Since finding out the truth about One For All, Bakugo has surprisingly been an important asset. It didn’t seem like a promising situation at first -- after all, he and Deku have never been on the best of terms. He’s done a great job of keeping it a secret though, not just for Deku’s sake but for All Might’s as well. Recent developments in Season 5 have further proved that Bakugo's discovery was a positive.
Deku has manifested a new Quirk, Blackwhip, and is set to unlock six more. Since All Might no longer possesses One For All, he cannot be involved in the physical aspect of training Deku to control his new powers. As showcased in the final Joint Training match, Blackwhip is currently far from stable. Should the Quirk go out of control again, All Might would probably be hurt in its rampage.
Bakugo, on the other hand, is in peak physical form. Training with Deku is less dangerous for him as he is more than agile enough to dodge whatever surprise attack may come his way. He is also powerful enough to pressure Deku into bringing out his best during training. Their private sessions may not have yielded results with Blackwhip just yet, but the extra practice is great for them both.
Having another person to discuss One For All’s mysteries with is also proving invaluable. Since Deku is entering territory uncharted even for All Might, having a new perspective to ponder over the Quirk's secrets can only help. Bakugo may not look it, but he’s actually very intuitive. He’s already been able to find a link between One For All’s multiple new powers and All For One’s Quirk.
Working closely together could also improve the famously fractious relationship between Bakugo and Deku. It's already grown somewhat since Bakugo learned the truth about One For All, and they’re now on good enough terms to team up while still competing for the number one spot. Since both are aiming for the top, this cooperation will help them become better, more capable heroes in My Hero Academia.
