When Deku first inherited One For All from All Might in My Hero Academia, neither of them had any idea that he was the perfect fit for the Quirk. One For All was created from a stockpiling Quirk that All For One forced onto his seemingly Quirkless brother. His brother actually did have a Quirk, which had no function other than allowing him to transfer his Quirk to other people. These two Quirks fused, making the Quirk stronger each time it was cultivated and passed on to a new user.
However, anyone who had a Quirk prior to inheriting One For All died at a young age, unable to bear the weight of both. Deku, being Quirkless, would not only be able to use One For All without that drawback, but was also able to manifest the Quirks of his predecessors. In the manga, he has unlocked 5 of these additional Quirks. These !uirks have all helped Deku, but some are more useful than others.
5 Danger Sense: This Quirk Allows Deku To Sense Nearby Threats
Deku is already uncannily similar to Spider-Man, but his new Quirk Danger Sense seems just like the MCU hero's Spidey-sense. Deku inherited this Quirk from the 4th user of One For All. Danger Sense allows Deku to detect any threats that are nearby. Unfortunately, this Quirk doesn't seem to be that useful, as Endeavor had shown Deku that heroes are able to pick up on danger at lightning speeds without the need for this type of Quirk. In order for this Quirk to be efficient, the user has to have fast reaction times.
Knowing that danger is coming isn't helpful if Deku cannot avoid it. Deku has shown that he has room to improve in this area, as he still took a fair share of hits during his battle against Lady Nagant. The drawback of the Quirk surfaced when Deku first used it against Tomura Shigaraki; he experienced a sharp pain in his head when he sensed danger. This could be more distracting than helpful in the midst of a battle.
4 Fa Jin: If Deku Builds Up Enough Energy, He Can Be Faster Than A Bullet
Fa Jin was the 3rd user's Quirk. The Quirk is much like the same-named concept in Chinese martial arts, in which the user must build up energy through repetitive moments before unleashing it in a powerful move. Deku can choose to use only some of the energy, but if he stores enough, he can move faster than a bullet. He was able to outpace Lady Nagant's bullet, specifically, saving Overhaul by knocking him out of the way.
It was miraculous that Overhaul wasn't hurt, but he would have been grievously injured if Deku had crashed into him instead. In order to build up enough energy, the user has to buy enough time, as enemies are not likely to allow him to stand there and repeat the same movements over and over again. Deku is also relatively new to this Quirk, meaning that it will take some practice before he's able to use it efficiently.
3 Smokescreen: Deku Can Release Smoke From His Body
Smokescreen was the Quirk of the 6th user of One For All. Deku has yet to master this Quirk, as the previous user repeatedly reminds him that he releases too much smoke at a time. This makes it more difficult for Deku to see what he's doing, and the enemy can instantly spot him the moment he leaves his smoke. When using this Quirk against Lady Nagant, he threw away any opportunity he had to escape from the battle. Many of his long-distance attacks, which propel a significant amount of air force toward the opponent, would alert enemies of his location. His smoke is also incapable of spreading when it's raining.
However, this Quirk, when used in moderation, can conceal the right people from villains, and anything Deku throws outside of the smokescreen can create a diversion. It buys him time to think of another strategy in high-pressure situations. Sometimes, a hero needs some time to breathe, especially someone like Deku, who relies so much on his own intellect to strategize his way to victory.
2 Float: Deku Can Use This Quirk To Fly Through The Air
Deku inherited Float from Nana Shimura, the 7th user of One For All and All Might's mentor. This Quirk allows the user to remain suspended in the air. Although Deku uses Blackwhip more, Float is particularly useful when he needs to be in the air and has nothing to grab onto. He can simulate flying by using air force to propel himself, moving in a way similar to swimming.
When this Quirk manifested during his battle against Tomura Shigaraki, it was particularly useful to be able to attack from the air, where Tomura wouldn't be able to decay the ground and have the advantage. In combination with Blackwhip, Deku can make a large group of people float as well. All Deku has to worry about is visualizing flying within his mind. He needs to keep his desire to fly in his head in order to remain suspended for long periods of time. Other than this, the Quirk doesn't seem to have any other significant drawbacks.
1 Blackwhip: Deku Can Use Whips For Mobility And To Grab Onto Things
Blackwhip is a Quirk that was used by the 5th user of One For All. Like Danger Sense, Blackwhip also seems to be inspired by Spider-Man. Deku can create black tendrils from his body, and uses these tendrils to swing from building to building. This gives him a massive amount of mobility. He can also use them to grab people, often pulling civilians to safety or attempting to restrain villains.
Deku analyzes the movements of Hanta Sero, Eraser Head, and Froppy, then replicates their movements. He can mimic Froppy's tongue by shooting a tendril out of his mouth, which he does when he cannot use his arms. Since the tendrils can extend from any part of his body, it gives him a lot of flexibility when it comes to using the Quirk. Making use of Blackwhip requires Deku to have control over his emotions, as the power can go out of control when he gets too angry. Like Fumikage Tokoyami, losing control makes him stronger, but also makes him a threat to his allies.