My Hero Academia is well known for playing jump rope with the concept of heroism, as many of these superpowered "good guys" have let all the fame and fortune get to their heads. Many villainous organizations are attempting to revolutionize the current social system to make things free and fair, as they believe the heroes are essentially running a tyrannous regime.
This epic Shonen pointedly convinces viewers to sympathize with multiple criminals and supplies several dislikeable heroes to create more angsty contrasts. The following heroes walk a fine line between good and evil, particularly when considering each Quirk's abilities and their individual personality types.
Katsuki Bakugo
Beginning with the obvious choice, Bakugo's erratic behavior during the Sports Festival showed so much potential that the League of Villains tried their absolute best to recruit the hot-head, but thankfully, to no avail. With his destructive powers and anarchist attitude, even Bakugo's fellow classmates are nervous around the boy and rarely view him as a true ally.
Anger, resentment, and jealousy (coupled with a relatively violent upbringing) have all the makings for a perfect villainous origin story. Bakugo only really began demonstrating heroic characteristics from Season 5, but up until then, he had most viewers worried he might just take a trip to the dark side.
Shihai Kuroiro
Shihai already has the perfect look for a nightmarish miscreant, and his hostile personality isn't exactly winning over many hearts either. Literally categorizes as a "Scheming Hero," Shihai is infamous for his deceptive strategies and has already proven to be quite efficient at mind games during the Joint Training Battle.
Not only can Shihai practically render himself invisible by disappearing into the shadows, but his Quirk allows him to travel through the gloom and virtually teleport to another dark location. Vantadark would not even need a name change to comfortably assimilate among the villains; he is all set and ready to go!
Kinoko Komori
While this cutie pie's personality type doesn't exactly scream evil, Kiniko's Quirk has some lethal properties that Dr. Ujiko probably has set high up on his wishlist! Her field of toadstools may be more annoying (and apparently, gross) than debilitating, but her schizophyllum commune fungi produce significantly more sinister results.
By summoning these seemingly innocent mushrooms, Kinoko can subdue her victims with their aggravating spores which overwhelm the lungs, leaving contaminated foes gasping for breath. If used at full capacity, Kinoko's Quirk has potentially fatal consequences, and even the slightest infection needs immediate medical treatment to provide relief.
Neito Monoma
It is hard to believe that the narcissistic Neito would be missed by many if he deserted UA High School, as he was likely only accepted into the Heroes Course due to his epic Quirk. Neito is lacking so much substance personality-wise that it is nearly impossible to even picture him saving the day for anyone, as he is arguably the most selfish student in My Hero Academia.
In a sense, Neito is like a dulled-down version of All For One, with the ability to copy someone's Quirks and even store a few selections within his system. He is a quick thinker and highly manipulative, already possessing the perfect blueprint for Shigaraki to potentially build upon.
Hitoshi Shinso
Shinso has all the makings of a legendary supervillain capable of bending pretty much anyone to his will. He is not often accused of being especially pleasant, however, his manipulation tactics are one point (which is often necessary to facilitate his Quirk's brainwashing activation.) Unfortunately for Shinso, people have treated him as a villain his whole life, and proving them right wouldn't take much of a stretch of the imagination.
The discrimination that this duplicitous young man constantly receives has led him to become a rather bitter and resentful person who seems to despise Midoriya (and, well, everyone else) from the moment they met. Shinso flat-out declares that he is not at UA to make friends, and his character design is even believed to have been strongly influenced by Marvel's supervillain, Purple Man.
Fumikage Tokoyami
This Prince of Darkness already has his foot in the door, as Dark Shadow is always ready to cause chaos at the first opportunity it gets, with zero concern for anyone hurt in the process. Poor Fumikage spends a great deal of energy trying to tame the sentient beast within; imagine what he could accomplish if behaving heroically was no longer of concern?
It also makes sense for villains to do most of their work under the cover of darkness, the perfect condition for Dark Shadow to thrive! Whether or not Fumikage's new (hypothetical) evil allies would survive the ordeal is debatable, but All For One could certainly find a way to manipulate the situation to his liking.
Romero Fujimi
This Isamu Academy student features in My Hero Academia's OVA titled "Training of the Dead" whereby he turns a crowd of students into a hoard of irrational zombies (who still have access to their Quirks.) Romero demonstrates no ability in controlling his monsters, which quickly creates a chaotic scene and would be the perfect theoretical counter to say Team Endeavor's Hospital Invasion in the Paranormal Liberation War Arc.
Romero shows no regard for the other student's safety (yes, even his allies) during the survival course and unleashes his death-defying zombification Quirk without assessing the situation beforehand. Much like Bakugo, this blue-haired boy loses control quite easily when angered which subsequently minimizes his use of self-restraint when releasing the toxic pink gas.
Seiji Shishikura
Seiji has the perfect personality for treachery while his physics-defying Quirk would be a great addition to All For One's squad, as it essentially turns any living organism into a meaty ball of putty. This Shiketsu High student went all out during the Provisional Hero License Exam, however, he seemed to simply enjoy the unrestrained abuse of power and makes no effort to actually pass the exam.
Seiji looks down upon those he deems unworthy but sees no merit in practicing self-reflection himself; so he is, therefore, unlikely to suddenly dig up any redeeming qualities. He is evidently very loyal, however, the same could be said for Shigaraki, Twice, Spinner, and several other scoundrels and is therefore not indicative of Seiji's alleged heroism.
New episodes of My Hero Academia Season 6 are released weekly on Crunchyroll.