All For One

Sensei (先生) is the true leader and founder of the League of Villains and is the true main antagonist of the series.

Appearance
Sensei's face has a completely deformed head from above his upper lip he has a massive scar that also extended behind his neck, while also missing both his eyes and nose. He also has two tubes in his throat that allow him to breathe.

In flashbacks, he appears to be an ordinary looking man with short hair. However, his face is not seen.

In each appearance to date, he has always been seen wearing a suit.

Personality
Sensei has a sinister, twisted and methodical personality, where he can coordinate many plans to work in conjunction with one another.

History
The introduction of Quirks to the world led to a period of societal collapse and an age of upheaval. During this time, a man (who would become Sensei) with an overwhelmingly powerful Quirk called All For One, arose in Japan and decided to take advantage of the chaos. He stole many Quirks which gave him immense power and influence. Using his incredible might, the man gathered and unified dozens of people under his command. He then manipulated the people he gathered like pawns to further his plans and eventually became Japan's ruler. It's suggested that he ruled from the shadows, as most of his actions in the present day are only known as rumors on the internet.

Sensei's younger brother opposed his evil. For unclear reasons, Sensei used All For One to forcibly give his little brother a Quirk that could stockpile power. What no one knew at the time is that Sensei's younger brother already had a Quirk with only the ability to pass itself on to others. The two Quirks mutated, blended together and would later become known as One For All.

Sensei's younger brother was not strong enough to defeat him with his new Quirk, so he passed it on to the next generation in the hopes that one day One For All would be strong enough to defeat his older brother. It is suggested that Sensei would fight these successors.

Later on, Sensei killed All Might's predecessor Shimura. All Might eventually fought and defeated Sensei, but received a debilitating wound in the process.

Incapacitated by his defeat, Sensei went into hiding as he attempted to heal from his injuries. He would later create the League of Villains (where he took on his "Sensei" identity) and appears to be grooming Tomura Shigaraki as his successor.

Unforeseen Simulation Joint Arc
Kurogiri warps himself and Tomura back to their hideout at the last second, allowing them to escape from being turned to dust. Tomura complains that his body hurts and all their subordinates were eliminated, Noumu was defeated and All Might is still alive, further complaining that everything went wrong. However, Sensei, speaking on the monitor says that nothing went wrong and that he was simply over optimistic. Sensei says that it is too late to mourn and that they should gather a new group hand picked for efficiency, saying that they should take all the time they need. Sensei says that Tomura is a symbol of importance and that next time he will show the world the horror of his existence.

Sports Festival Arc
A recuperating Tomura was seen watching the Obstacle Race with his computer and he sees that Izuku took first place. Later on, before the battle between Izuku and Shouto begins, Sensei tells Tomura to pay close attention to Izuku and Shouto's match as they might become obstacles for him in the future. Tomura responds by telling the Sensei to not make him laugh.

Field Training Arc
Sensei, using the monitor to speak through, orders Kurogiri to allow Stain and Tomura to continue talking, saying that Tomura's meeting with Stain (an experienced infamous villain) will stimulate Tomura's growth. Before he leaves for Hosu, Tomura asks Sensei to let him borrow some Noumus because he doesn't like the Hero Killer Stain and that destroying things he doesn't like seems alright. Sensei decides to give Tomura three Noumus, hoping that Tomura will make the opportunity a learning experience.

The Sensei is surprised that the Hero Killer Stain was captured but is still happy that things are going as he has foreseen. He exclaims that Tomura's position is to supervise and unify the new villains who have come to join the League of Villains. The Sensei's doctor expresses doubt that Tomura can do his job and says that if Sensei stepped in things would be proceeding faster. Sensei asks the doctor to patch him up faster. Sensei says that he will let Tomura do all the hard work so that he may become his successor as Tomura was born to do so. Sensei tells All Might to rejoice in the "transient" peace while he still can.

School Trip Arc
After Katsuki attacks Tomura, Tomura understands that mere words alone will convince Katsuki to join them and decides to ask for the “force” of his teacher, Sensei, to help him convince Katsuki. Sensei, who has been watching Tomura on his computer, commends his apprentice for his wise decision.

Quirk and Abilities
Little is known about Sensei's prowess. However, it can be assumed that Sensei is extremely powerful as he battled All Might when he was in his prime and gave him a grievous wound (which would limit All Might's usage of One For All).

All For One: Sensei's Quirk allows him to steal people's Quirks and renders them the user's own. This Quirk can also transfer and grant Quirks (stolen by All For One) to other people.

Battles

 * All Might vs. Sensei: Draw