One of the best things about My Hero Academia as a narrative about superheroes and a superpowered society is the character of Tomura Shigaraki (Quirk: Decay), whose development in the series from small-time baddie to cataclysmic threat has occurred in parallel to Deku's slow mastery of One For All. The character's relevance to the My Hero Academia story, as well as the fated connection between himself and the Quirks All For One and One For All, and his ascension in the latter parts of the series have made him one of the series' best-developed characters.
Shigaraki's development gets particularly interesting after the retirement of the former Number 1 hero, All Might (Quirk: One For All) in the aftermath of his brutal battle against the main antagonist of the franchise, the wielder of All For One, and now having reawakened in the current War Arc, Shigaraki has become the biggest threat the heroes have faced since All For One. Adopted by All For One as a child, Shigaraki's origins were a mystery until the My Villain Academia arc which was a part of the anime's 5th season. Here's why Tomura Shigaraki is rapidly becoming an example of the ultimate villain in shonen anime.
Introduction
Tomura Shigaraki (Real Name: Tenko Shimura | Quirk: Decay) is the series' introduction to organized villainy in the My Hero Academia universe as he is first introduced leading a group of discontent individuals in a faction known as the League of Villains. They were revealed to be directly affiliated with the biggest villain in the world, the immense threat who permanently injured All Might in their last encounter – All For One. Shigaraki's relationship with All For One has always been shown to be that of master and student, and it quickly becomes apparent that the series' primary antagonist is looking for a way to develop a new generation villain to follow in his footsteps.
This is quite obviously the series' contrast for Deku's inheritance of All Might's abilities in the One For All Quirk, and the series narrative is slowly but surely revealed to be a contestation between the original wielders of either Quirk, with One For All being passed from generation to generation with the hopes of it one day evolving into the kind of multi-faceted Quirk capable of dealing with the sheer oppressive power of the sheer number of Quirks stolen with the All For One ability. This makes Shigaraki the dark mirror to Izuku Midoriya.
This major threat to the world is introduced as having been slowed down, but not subdued after his last battle against All Might, which left him gravely injured. Shigaraki is hence All For One's solution to the Deku problem, and his next means of trying to absorb One For All – once and for all. Shigaraki makes his series debut as the primary antagonist of the U.S.J Arc, in which he leads a faction of rogues in an invasion of U.A. High.
The invasion followed the news that All Might joined the school's faculty. The "U.S.J. Incident" as it came to be known was an assassination attempt on All Might orchestrated by Shigaraki and his League of Villains. This event is the series' introduction of one of the most indomitable threats to the heroes, the Nomu. While the attack fails, Shigaraki grows as the series progresses, and it is the final leap in his development that turns him into a major threat.
Completely Broken Combat Ability
Shigaraki is already a huge danger to society prior to his awakening and the subsequent transplant of All For One and all associated Quirks into his body. His original Quirk, Decay, is capable of disintegrating anything he touches with all five fingers, whether the target is organic or not. This disintegration travels from the touch site, and will engulf the victim's entire body if they do not get rid of the decaying body part – meaning that depending on the situation, Decay is almost always fatal. One of the Quirk's weaknesses is that it travels through solid matter and cannot affect liquids and can thus be countered by any particle manipulation Quirk.
His initial awakening in Deika City saw his Decay Quirk evolve and become lethal even when Shigaraki hadn't touched the target. However, after having the All For One Quirk inserted into his body among others, Shigaraki's combat abilities shoot far beyond anything anyone has ever seen before. Not only are his Quirks a major threat, but he has become monstrously strong in other respects. He can jump further, climb higher and hit harder, all while being able to rapidly regenerate; see the location and weaknesses of his enemies, and cause instant death. Upon seeing an unconscious Shigaraki in his bio-canister prior to his awakening, the Number 5 Hero, Mirko, experiences a feeling akin to the instinct of a rabbit being hunted by a predator in the wild. Her instinct as an animal let Mirko know that the entity within is not one who should be allowed to awaken.
Origin
During the My Villain Academia arc, which happened during season 5, the true origins of Shigaraki Tomura were revealed. This showed how the unfortunate child, Tenko Shimura, caught up in family trauma caused by the heroics of his grandmother, Nene Shimura, the wielder of One For All who mentored Toshinori Yagi (Hero Name: All Might | Quirk: One For All). Shigaraki's father was explicitly anti-hero after his mother, Nene, left home to focus on fighting crime. The reasons behind Shimura's decision to abandon her family, while unfortunately unavailable to her family, are quite apparent as it is well understood that as a vestige or wielder of One For All, being a hero was secondary to preparing the self for the unavoidable battle against All For One. This locks anyone who possesses One For All in an inevitable battle akin to that of Bast and Apophis. Due to the trauma of being abandoned, Shigaraki's father became hateful towards the concept of heroes, and didn't consider heroes to be inherently just or virtuous because his mother abandoned her family to be a hero.
Despite this, Shigaraki wanted to become a hero as a child; however, a traumatic event involving a lie told by his sister and abuse from his father saw 5-year-old Shigaraki get locked outside with the dog. As a child, he suffered from intense itching and dryness which eventually became the Quirk known as Decay. It unfortunately first manifests while young Shigaraki is embracing his dog, and had slowly been progressing prior to its awakening on that fateful day. Young Shigaraki ends up murdering his family, destroying his home with his Quirk and living on the streets for a while before being picked up by the series antagonist and nurtured to become his successor, the new Symbol of Fear.
Connections
What makes Shigaraki particularly compelling as an antagonist goes beyond his actions within the narrative. His position in the My Hero Academia narrative as the dark foil to Izuku Midoriya, while also having an intricate connection to the main character, the main Quirk in the series and the original antagonist of the series makes the pay-off regarding Shigaraki's origin and awakening during the My Villain Academia arc truly enjoyable. Centrality to the narrative makes an antagonist even more compelling – this is evident in one of the Dragon Ball franchise's most iconic villains, Frieza. Due to his grandmother being a wielder of One For All, Shigaraki's home life was destroyed and he was subject to various kinds of abuse and discouragement.
Like the young heroes in the series, he also had a great admiration for professional heroes and aspired to be one, but a poor upbringing in an unhappy home led Shigaraki down the darkest path very early on in his life. He had no chance. His connection to Nene Shimura, the predecessor of his mentor's greatest enemy, All Might, creates a level of irony as the family of one of the most important heroes became the incubator of one of the world's most dangerous individuals. Shigaraki is an incredible shonen villain because he is feared in his respective universe, is deeply connected to the most significant characters in the narrative in a way that employs a kind of tragedy in Shigaraki's characterization to give credence to his hatred of the status quo and fealty to evil. This, in addition to chilling character design, brilliant depiction in the anime and an arc that is drawn-out but has great pay-off, make My Hero Academia's Tomura Shigaraki an ultimate and exemplary villain in shonen.