- It's often believed that pro heroes are prone to have psychological issues, most commonly nightmares or insomnia. Some even go as far as to develop PTSD.
- Although so far the manga has been pretty lacking in defining how a hero acts during a police investigation, whether it's participating in the investigation or just in the arrests, most fans believe the former to be true, and many fanfics run with this premise, having heroes take active part in investigations alongside detectives like Naomasa. This perception is supported in canon by the fact that Endeavor demonstrates the analytical skills of a detective during the Field Training Arc, and would explain why his number of resolved incidents surpassed even All Might's. Illegals would later confirm this as canon.
- It is commonly believed that the robots during U.A.'s entrance exam had a hidden button that deactivated them, explaining how certain characters like Hagakure and Koda passed the exam. However, this is never indicated in the manga. The closest to this is that by Word of God, Mineta managed to pass by using his Pop-Off Quirk to restrain the robots rather than destroy them, meaning the robots didn't have to be destroyed to be beaten.
- Many fans assume that hundreds of years have gone by since Quirks first appeared. While there are definitely futuristic elements in the setting like robots and holograms, they're in small supply and the setting is largely 20 Minutes into the Future, with no exact timeframe from the emergence of Quirks to present day ever given.
- While the subject of killing your enemies has never actually been addressed, many fans believe that heroes are expected to practice a Thou Shalt Not Kill policy at all times and that killing is an ultimate taboo that only villains do.
- The fandom believes that there is a "Bakusquad" and a "Dekusquad". The former is a group that usually hangs with Bakugo composed of Kirishima, Sero, Kaminari and Ashido, while the latter is the group that hangs out with Midoriya, composed of Iida, Uraraka, Todoroki and Tsuyu. While it's not farfetched to believe these characters to be friends, they are never actually seen hanging out in these groups.
- Drawing future versions of the main characters is very common. Midoriya's future self, however, has been the most popular to draw and many fans have common views on some things:
- He will cut his hair in the future, while a lot of people like to show him with an undercut, some fans think he will just tame his bush of a hair. To a lesser degree, some think he'll just style his hair and end up looking like Spike from Cowboy Bebop.
- His updated hero costume will include a cape and there will be more black details.
- It's a common misconception that Midoriya is faster than Iida when using Full Cowl, due in part to the Recap Episode of Season 3 where the two raced and Midoriya won. However, said episode was anime only bordering on Filler, meaning there is no canonical evidence Midoriya is faster. Quite the opposite in fact, as one chapter of the Internship Arc had Iida running on water, a feat that would need Iida to be running 30 meters a second, whereas Midoriya, who was admittedly distracted, was being washed away by the waves.
- Midoriya is frequently depicted in fan art with tons of scars running up and down his arms and across his chest and back. In the actual story, his scarring is almost entirely restricted to his right arm.
- Midoriya is commonly depicted as having a rabbit-themed Animal Motif and/or being compared to a rabbit by others, due to the way his hood resembles rabbit ears more than it resembles All Might's hair and the fact that he wears a rabbit outfit in My Hero One's Justice. When he is portrayed as an anthropomorphized animal, he is more often than not a rabbit.
- Midoriya is often portrayed as completely unable to cook, even the most basic dishes or his favorite one, Katsudon. Some fanfics of him as an adult have him having to rely on his mother, his current romantic partner or convenience store food to eat and have questionable eating habits, since he can't make proper meals alone. Some other depictions extend this to poor sleep schedules and being an overall huge Workaholic to the detriment of his health.
- To some fans, Bakugo is actually very remorseful about his treatment of Midoriya over the years, which shows up in fics where their relationship (platonic or not) is in the forefront.
- After Bakugo is shown to be able to cook, some fans like to portray him as a downright Supreme Chef in the kitchen.
- When Bakugo's hero name needs to be mentioned in fanworks, fans usually go with Ground Zero. This stems from the fact that Bakugo's early concepts said that that was his hero name.
- It's common to see Bakugo portrayed as going out of his way to bully Midoriya throughout the entirety of his childhood once they stopped being friends. However, it's implied Bakugo mostly ignored Midoriya, considering him unimportant until he put himself back on Bakugo's radar by wanting to go to U.A.
- We've yet to see Bakugo's dorm room at U.A., but most fans speculate he has a Dart Board Of Hate with Midoriya's picture tapped to it and a varying number of darts in it. An alternate theory suggests it is also extremely neat, as he makes sure that all dirt, mess, and other such things die!
- Much like how Midoriya is believed to have a Rabbit Motif, Uraraka is believed to have a space motif, in part because of her Quirk, "Zero Gravity", and other because of her appreciation for the space motif-ed hero Thirteen, as well as some interpretations that her helmet was meant to emulate a space helmet. A lot of fan art depicts her being surrounded by space or acting as an astronaut.
- It's usually believed that Todoroki no longer has sensation in his burnt scars, or has very little, not enough to feel anything but slight touches. This is usually brought up in slash fics between him and Midoriya.
- Some fans believe that Todoroki's left eye is blind, due to the burning and sometimes as a result of mistaking his heterochromia (as flashbacks show his left eye has always been blue) as a cataract. Similarly, fans also believe that he is actually quite hairy and prone to have a beard due to his father's appearance and that his body hair is red on the left side and white on the right, following his hair pattern.
- Kirishima is a very average Nice Guy, if a bit boisterous, but several fanfics like to portray him as having something of a Frat Bro-like personality, with a tendency to call his friends "bro" almost all the time. In canon, he has never called anyone "bro", or any nickname for that matter, and tends to just call people by their name.
- Kaminari is often portrayed as being a good friend of Bakugo in fan art due to his friendship with Kirishima, even though the two of them have never really interacted outside of their scene together at the start of the USJ Arc where Kaminari says that like the majority of his classmates, he dislikes Bakugo. In Chapter 107, Kaminari shows that he respects Bakugo, and considers him one of his mates, so it might be a case of Ascended Fanon.
- Many fans exaggerate Mineta's status as The Friend Nobody Likes. While it's true that some of his classmates (especially the girls) dislike him for his perverted behavior, Midoriya and Tsuyu are shown to get along with him, and he's friends with Kaminari because of their shared interest in girls. The rest of the class does manage to be nice to him every now and then, and in turn he's shown to be on good terms with them as well, despite how much of a pervert he can be.
- Ashido's first choice of hero name, Alien Queen, is a lot more popular then her current alias of Pinky. In fan art, fanfic, and discussions, it's far and away more common to see her referred to as Alien Queen then not.
- In fanworks, Tsuyu will often catch flies with her tongue and eat them like a frog would. The number of times she does this in either the anime or manga is pretty much zero. And when she's shown eating, she actually eats normal food like everybody else.
- Due to his eye bags, it's generally believed that Shinso has some sort of sleep disorder, specifically, insomnia, but darker fics tend to also give him things such as sleep paralysis.
- Monoma's rivalry with Class 1-A is often believed to extend to all of Class 1-B, they just don't make it obvious. This is, however, false, as while there is some Friendly Rivalry amidst the two classes such as Kirishima and Tetsutetsu, or Yaoyorozu and Kendo, for the most part the two classes are amicable. Additionally, Class 1-B generally finds Monoma's antics with Class 1-A to be annoying and/or concerning.
- According to Word of God, Ibara Shiozaki was originally slated to be a member of Class 1-A. Fans mistakenly believe this was changed due to Horikoshi finding her hair difficult to draw, as both statements came from the same character brief. However, Horikoshi simply stated in the brief that he couldn't remember the exact reason, something that would be unlikely if it was connected to the aforementioned hair issues.
- No real reference is made to Mirio's size, but due to his shamelessness when his clothes drop and status as Mr. Fanservice, the general belief is that he has a Gag Penis.
- More often than not, All Might's "Muscle Form" is believed to be a result of his using One For All, and has in turn caused the idea that Midoriya will also gain a similar state. However, All Might's "Muscle Form" was his original natural state as a result of training and using One For All. It was only after he suffered serious injuries at the hands of All For One did he become emaciated, and he could only restore his old form with the embers of One For All, not because of it being a power of One For All, but because, as he put it, its like flexing and puffing out his chest.
- Fans believe that the group of adult heroes in the manga have all studied in U.A. together. Particularly, All Might, Aizawa, Present Mic, Midnight, and Endeavor. The relationship between All Might and Endeavor would also have been similar to Bakugo and Midoriya, but Endeavor would openly want All Might to join him so they would become stronger than anyone else. There is something to this theory; Aizawa and Present Mic were in the same class, and Mic was the one who came up with Aizawa's hero name, Eraserhead. Some fans have also believe that Endeavor's obsession with All Might started in high school, with some fall out between them causing it. Shippers think that they are exes that ended on poor terms. However, canon indicates that All Might was already the number one hero when Endeavor started his career (combined with All Might going to America to start his hero career and attend college immediately after graduation from U.A.), and Midnight refers to All Might as an "older man", so it's unlikely they went to school at the same time he did.
- Some believe that Tensei Iida, Midnight, and the duo of Aizawa and Present Mic were in the same class due to all of them being the same age. This can extend to them watching/babysitting Tenya when he was growing up, and Tenya knowing them on a personal level but choosing to stay professional while at school.
- Some have taken to believe that All Might, due to the huge amount of Western comics Homage he shows in design and personality, is of American/Japanese Mixed Ancestry and genuinely has ties to the USA, either through family or being raised there. Volume 0 and Two Heroes showed his backstory was a little different; All Might was originally from Japan, but he lived in the U.S. for some time after graduating, and began his hero career there; hence, the reason why all of his signature attacks are American-themed.
- Midnight is portrayed as having a pretty active and casual sex life and is the one relegated to teaching sex ed to U.A. students. Some shippers like to add Aizawa, Mount Lady and/or All Might as one of her casual partners.
- Some fans believe Aizawa used to be in Gen Ed studies before moving to Heroics department due to his Quirk not being particularly useful during the entrance exam and his general distaste for it.
- Snipe is thought to have still gone through some version of the events of the My Hero one-shot, with him getting a job at U.A. after the end. To a lesser extent, he keeps in contact with Jack, who himself is usually headcanoned as part of the Midoriya clan or an Identical Stranger to Deku.
Main Characters' Families
- Inko is sometimes portrayed as resentful towards how Bakugo treated Midoriya over the years, and only wouldn't have taken any action because her own son kept stopping her and creating excuses for him.
- For many fans, Inko and Mitsuki were childhood friends, with their relationship being different to that of their children, with Mitsuki being a Hot-Blooded Japanese Delinquent, while Inko was the Cloudcuckoolander's Minder who stopped her from getting into fights. Some also add Masaki in the mix and showcase them as high school sweethearts.
- Hisashi Midoriya is frequently portrayed as being almost totally absent from his son's life, to the point that Izuku can't even remember his face. While this is somewhat justified by Hisashi's status as The Ghost in the story, it's more than likely that there are at least a few family photos of him lying around the apartment.
- Before Toya's name was revealed, many fanfic writers opted to call him as Ryota Todoroki or the short form, Ryo. Additionally, the fandom has more or less completely accepted the idea that Dabi really is Toya Todoroki, though this one may yet end up confirmed for the very reasons it's already considered canon.
- It's unknown what Quirk Naomasa Tsukauchi has, but many fanworks lean to his Quirk being a Living Lie Detector Quirk in relation to his job as a police officer and also because of his sister's Quirk being a Living Lie Detector as well. Just as likely is for Naomasa to be depicted as Quirkless, due in part to All Might's initial suggestion to the then Quirkless Midoriya be to join the Police Force.
- Many believe that the 20% figure for proportion of Quirkless people refers to the entire population in the present time, but not to the current birth rate. This is evidenced by Midoriya being the only Quirkless person in his middle school class, not to mention the number of truly Quirkless characters overall being countable on one hand. In addition, All Might's comments about the status being less unusual in his youth suggest a changing demographic over time.
- A rather dark one, but there's a common belief that Endeavor forced his wife to have their children for his ambitions. While this is understandable to assume given his abusive tendencies, it hasn't been outright confirmed in canon.
Other Media Characters
- Some fans believe that Koichi, the protagonist of Illegals, continued a friendship with Tensei Iida well after the events of that spin-off. A lot of fan art depicts them meeting after Tensei's accident and Koichi often visiting Tensei in the hospital and afterwards.
- The Transplant characters from Horikoshi's previous work are sometimes believed to know each other.