My Hero Academia’s Pro Hero Sir Nighteye could have made Izuku Midoriya into the perfect hero, but their time together was cut short.

Protagonist Izuku Midoriya and Sir Nighteye got off to a rocky start in My Hero Academia, yet their relationship was tragically cut short just as it was starting to flourish. Nighteye disapproved of Deku as All Might’s chosen successor, so much so that his and the latter’s lengthy friendship fell apart over the cause. However, over time Midoriya proved himself to be more than worthy of the title.
As the pair were beginning to bond and Nighteye saw Deku’s true capabilities, the hero was fatally wounded while fighting the villain Overhaul. This death was tragic in many ways; Midoriya lost a mentor and All Might lost his closest friend, but Sir Nighteye’s brimming potential as a teacher for Deku was also lost.
Sir Nighteye Knew Everything About All Might in My Hero Academia

Sir Nighteye was All Might’s biggest fan in MHA, with his knowledge of the #1 Pro Hero surpassing even Midoriya’s. On top of that, he was the hero’s sidekick for years and was his closest friend and confidant. With this knowledge, there’s no doubt Nighteye knew exactly what All Might did to ascend to the top Pro Hero spot. Using this inside information, he could have passed his wisdom down to Deku and trained him in the same way All Might did.
Considering the pair have the same power, training their strength similarly would be ideal. All Might was never really able to replicate his own regime when helping Midoriya, so Sir Nighteye would have been his best bet at replicating the hero’s fighting style. While the young apprentice is a good fighter, he’s never been formally trained in combat, so Nighteye could have honed these skills using his knowledge of the battlefield. He could have taught Deku ways to apply all his strength, making him a less predictable and an overall more capable fighter.
Sir Nighteye’s Wealth of Experience Could’ve Helped Deku Grow

Part of why Sir Nighteye would have been such a great teacher for Midoriya stems from the experience the hero had. Not only was he well-versed in the ins and outs of combat, but Nighteye also proved himself to be an excellent mentor before. He previously guided Mirio Togata and deemed the young hero to be his greatest success. Considering a good portion of Mirio’s success came from Sir Nighteye’s teachings — which turned him into one of UA’s top students in under a year — there’s no doubt Midoriya would have been in good hands.
Due to being a well-experienced hero, Sir Nighteye could have also taught Deku about the side of hero work that goes unseen. He would have given Midoriya invaluable information about the values heroes should have, as well as the procedure of hero work. This could be everything from taking care of the paperwork, to how to approach a rescue situation or how to best present himself to other people — all of which Midoriya doesn’t have the best grasp on. Nighteye could have even taught Deku the subtler arts of hero work, such as spying on people, gathering intel and how to be as observant as possible.
Sir Nighteye would have ultimately been an excellent mentor for Deku in My Hero Academia, teaching him everything he’s currently lacking and perfecting all the young hero’s rough edges. With his guidance, there’s no doubt Midoriya would have been a completely unstoppable force in current arcs.