Overview
Izuku, or Deku if you go by his chosen superhero name, was once a Quirkless (normal) human boy. Through a confluence of circumstances where he demonstrated his heroism despite his fragility, he impressed his long-time idol and was granted the rare honor of One for All, a Quirk that can be passed down to others by the previous user, and cultivated to be stronger with each new hero. Using this power, he gained entry into UA Academy, a prestigious Pro Hero school that teaches children about the rigors of being a hero to prepare them for that line of work. His powers grant him incredible physical prowess, turning him into a powerhouse during any crisis, but they can also put an incredible strain on his body. And since he is so obsessed with his goal of being a Pro Hero, he overlooks the limitations of his own body, and acts without thinking. He still has a ways to go before he can be the Number One Pro Hero in his world.
Perks
One for All< Edge >: Izuku's Quirk, an activated superhuman physical enhancement.
One for All is an ability that allows its user levels of physical strength far and beyond the levels of a typical human being, up to and including punching something so hard that the air pressure changes, and leaping miles in a single bound. Unfortunately, more than the bare minimum usage of this Quirk can create what is best described as a "bomb going off inside his body," because he was not born with and it came pre-cultivated at its full power, giving him none of the intuitive limitations to stop it from shattering his body from misuse. That doesn't always stop him when things get real bad, though, and he's prone to using it at 100% power when the alternative is failing to be the hero he wants to be.
Full Cowl: The limited, everyday usage of One for All.
By activating One for All at about 2 to 5% throughout his body, Izuku can be fairly effective in a fight, allowing him to leap higher, move faster, and hit harder than the typical human could at peak capability. This lets him stand toe-to-toe with most low-level threats, and pose a measureable challenge even to more dangerous opponents.
Defensive Combat: Izuku has an adaptive fighting style.
Deku focuses on defensive combat during a real crisis, to mitigate harm to himself and to innocents in his vicinity. To accomplish this, he studies information about his opponents to get more familiar with their strengths and weaknesses, and he carefully observes opponents he isn't yet familiar with to learn how he should fight them. While he isn't perfect, and this reactionary style can use some work, it allows him to fight more dangerous opponents without getting himself killed.
Hero Fanboy: Deku has a knack for teamwork, thanks to his love of superpowered heroes.
Izuku has an intuitive mind for the capabilities of superpowered individuals. He keeps a series of journals to document the powers of many Heroes, as well as the individuals he meets out on his Internships. It gives him a very keen ability for teamwork, as he is able to utilize the abilities of others when they might choke during a fight. If a hero or villain has any sort of public presence, you can count on the very high chance he knows a lot about them already, as he's a very dedicated study of the world of superpowers.
Determined: Izuku's stalwart hero instincts grant him an absurd level of willpower.
Izuku is almost insanely determined to be a hero. This goes up to and including breaking free from powerful mind control through the power of his own very human will, and ignoring physical damage up to entire limbs being shattered like glass. This doesn't mean he is immune to fear, but it does means fear does not stop him from acting. He will be a Big Damn Hero all the way until he can no longer move.
Complications
Impulse: Izuku goes out of his way to pursue his ideals even when he shouldn't.
While Izuku is a very good kid, he also has a habit of acting before he thinks. He will leap into danger involving people much stronger than him, because the alternative is sitting by and watching people get hurt, even if it will end up with him getting seriously injured instead. He will continue acting even when doing so might further injure him, and he might go to pursue villains and other threats without the permission of his mentors and teachers, landing him in hot water down the road.
SMASH!: One for All was not a Quirk meant for the inexperienced and reckless.
Izuku's Quirk is explosively dangerous for his body. Overuse of it can, and will, render his body useless in the long-term if he's not careful, and in the short-term it can result in a lot of needless self-destruction since he has a poor grasp of his limitations or how to even use the Quirk effectively, forcing him into hospitals to recover from damage he has done to himself on accident. Further, he has a tendency to mimic All-Might, going for thunderous finishing punches on really dangerous opponents, which can shatter his entire arm, accelerating the risk to his body's integrity.
Idol Worship: Deku may forget that superpowered people are just as flawed as anyone.
All-Might, and by extension most superheroes and similarly superpowered people, are flawed people... and Izuku has a hard time interacting with them because of this. He's still young, and doesn't get how dangerous being a professional Hero really is, and what it means to carry that burden of responsibility. He doesn't understand the paperwork, laws, publicity, and more, beyond the most basic knowledge, forgetting that in order to be a Hero, you need to be accountable for your decisions. It can be abrasive to more professional mindsets dedicated to the job and not the ideal.
Fear: Izuku isn't ready to be a real Hero yet.
Izuku is afraid. He's afraid for others, he's awkward, he's unsure, he's prone to obsessing in order to improve to reduce risk to others, and most of all he is ruled by his terror during a real fight. He's yet to gain the fortitude to combat his foes and thwart disaster with a smile on his face, so in the end he's just an idealistic child with superpowers. Sometimes, it can all get pretty overwhelming, and it can reduce his ability to make smart choices.
Unlicensed: Izuku doesn't yet have authorization for official hero work.
Izuku is a student of the hero course at UA Academy, so he does not have the authority to act as a Professional Hero. This means he cannot actively engage in heroics unsupervised unless there is simply no other option, and doing so can land him in some hot water. The best outcome is that he gets a firm talking-to, the worst outcome is expulsion or even legal action against him. The rules of his world about vigilantism are strict, and he must be accountable to the laws of his world and any others he might visit and intern at.