Compared to most of his classmates, Tenya Iida has had a limited role so far in My Hero Academia’s final war. He first accompanied Shoto Todoroki to confront Dabi at Kamino ward, but after the Blueflame’s reawakening and subsequent warp to Gunga Villa, Iida has become one of the war’s most important participants. He was tasked by All Might to transport Shoto to his out-of-control brother’s location before Dabi could set off a devastating explosion. In doing so, Iida has finally recognized his purpose as a hero and completed his personal character arc.
No MHA character has reinforced the idea of saving others being a hero's most important quality more than Iida. His motivation for enrolling in UA was to become a hero as respected as Ingenium, his older brother. It was an objectively noble goal, but he began to have subtle doubts ever since he noticed Midoriya Izuku during the UA entrance exam. When every other candidate was rushing away from an enemy that wouldn’t give them any points in the exam, Iida saw Deku running toward it in order to save Uraraka Ochaco, and eventually get rewarded for his choice. Ever since then, he declared Deku his rival in order to understand what motivated him to take such a bizarre course of action.
Hero Killer Stain Forced Iida to Reevaluate Heroism
My Hero Academia's first and most damning indictment of Iida’s heroic ideals was the emergence of the Hero Killer. Stain, who had previously been little more than an urban legend, turned into a very serious menace to the major characters of the book when he attacked Iida's brother Ingenium, leaving him paralyzed and unable to continue acting as a hero. Iida, who was obviously devastated by the news, controversially decided to go after Stain by himself and, if possible, kill his brother's attacker.
Iida's battle with Stain, ironically, proved the Hero Killer's entire argument. Stain felt that only Pro Heroes who took on the mantle to save people were worthy of it, and that anyone who joined their ranks for personal gain or profit was greedy and deserving of extinction. When Iida went into Stain, the villain had already surrounded and was about to kill a hapless Pro Hero. However, because he was blinded by wrath, Iida chose to attack Stain instead of prioritizing the safety of his captive. Because his heroic character was so tightly linked to his brother's image, Iida's judgment had been corrupted, and by Stain's criteria, Iida deserved to die as well.
Iida Finally Understands the Purpose of His Quirk in My Hero Academia chapter 390
Of course, Stain didn’t explain the reasoning behind why he chose to dispatch Iida -- ignoring Deku and Shoto, who showed up later -- to Class 1-A’s president, but his choices were enough to inspire some introspection. Later, Iida’s dedication to ensuring all his teammates’ wellbeing during the Joint Training exercise was the only force that prevented the Class 1-B team from securing another win. It was a full-circle moment when he revealed to Deku that his strategy during that match was simply to emulate what he thought the protagonist would do in his shoes. He was slowly beginning to realize his Quirk had a lot more utility as a tool to save others than as a weapon of war.
Iida has been unconsciously using his Engine Quirk in this manner for much of My Hero Academia, but Chapter 390 proves he’s finally fully aware of his desired role as a hero. Whether it was rushing to alert UA school authorities to the villain invasion at USJ, activating Reciprio Burst to snatch a last-minute victory for his teammates during the Sports Festival or facilitating Bakugo Katsuki’s rescue during the Kamino Incident, Engine has always performed best when in service of others. It’s never had a better moment than going supersonic with Shoto’s assistance to save countless lives. Since his friend was present for many of these pivotal moments in his journey to discovering his heroic identity, Iida had to thank Shoto for helping him reach this stage while wishing him the best on his own journey.