WARNING: The following contains spoilers from Season 5, Episode 3 of My Hero Academia "Clash! Class A vs. Class B!", now streaming through Crunchyroll, Funimation, and Hulu.
After a long wait that lasted three seasons and included a share of sporadic appearances, Hitoshi Shinso returned to My Hero Academia in Episode 3 of the fifth season. It appears that Shota Aizawa/Eraserhead has taken the hero-hopeful under his wing, bringing him to join in with Class 1-A and 1-B for the Joint Training Battle.
Shinso reminded the first-year students of UA High School's hero program of his vow to overtake everyone and be the best hero, a humble aspiration cultivated from the Sports Festival in Season 2. However, for Izuku Midoriya/Deku, Shinso’s return prompted him to ponder his dream about the history of One for All from Episode 2, which served as a follow-up to their fight in the Sports Festival where he first saw the power’s vestiges.
In recounting his mysterious contact with the first wielder of One for All to All Might, Deku hypothesized: was Shinso and his Quirk the chief catalyst that unlocked his power? While various theories have popped up concerning Deku’s strange subconscious One for All manifestation, there is nevertheless reason to speculate Shinso had a hand in awakening it. After all, he was there when Deku first tapped in his dormant power.
Looking back to the aforementioned Sports Festival battle in Season 2, Episode 7, Deku was resoundingly trapped under Shinso’s Brainwashing and teetered on the precipice of elimination. With nothing on the table for Deku that could help him signal a comeback in the fight, he was unexpectedly assisted by the will of One for All, or rather, the eight other users who held the power before him. This event triggered the Quirk to activate, unsolicited by Deku.
Deku pointed this out to All Might in the fight’s immediate aftermath, much to his mentor’s agreement -- he essentially faced the same predicaments while harnessing One for All. However, Deku has now talked to the Quirk’s first user, something that All Might never did. Nevertheless, All Might stressed to Deku that had it not been for his strong emotional response to seeing the vestiges, then he wouldn’t have escaped Shinso’s Brainwashing.
Although Deku managing to see the previous users of One for All indicates his gradual accommodation with the power-building Quirk, that didn’t mean he was due to encounter them soon. Since experiencing the awakening of his Quirk’s transcendental capabilities in the fight against Shinso, Deku hadn’t undergone a second occurrence through his first year at UA until his strange dream. Strangely, his dream happened the night before he'd reencounter Shinso.
It has been a good while since Deku and Shino faced off at the Sports Festival. Still, both have had their own set of difficulties to face in their respective journeys to become heroes. Deku now meets new challenges with understanding One for All and the truth behind his power. Simultaneously, Shinso continues to disprove the negative stigma around his Brainwashing Quirk that so many people assume he'll use negatively.
If their previous encounter is anything to go off, then Deku and Shinso's upcoming clash in the Joint Training Battle is something to look forward to. With Shinso's hero aspirations and Deku handling the explosive power he wields, something has got to give when they come to blows in their forthcoming battle. Considering that Shinso's Quirk affects his target's mind, it may hold a unique key to unlocking more secrets of One for All for Deku.