Soga Kugisaki/Synopsis

History
When he was a young, Soga wanted to become a Pro Hero. However, people were constantly writing him off based on his appearance, his violent behavior and his dangerous Quirk, rejecting and ridiculing him as if he were some monster. This caused him to develop a violent personality and be rather sensitive to people laughing at him, getting into trouble frequently.

He also used to play guitar up until he got the one he owned destroyed in a fight.

Vigilantes Beginnings Arc
While he is walking with his friends Moyuru Tochi and Rapt Tokage, Koichi Haimawari unwittingly bumps into him and spills his juice. Wanting to humiliate Koichi, Soga asks him to apologize by kneeling at his feet, but Koichi flees. After that, he and his friends enter a convenience store, and they encounter Koichi again. Soga becomes angry at seeing Koichi again and takes him outside to beat him. Koichi is not able to fight back because of the spikes Soga produces with his Spike Quirk. Later that same day, Koichi and Kazuho Haneyama meet the thugs in an alleyway, and the thugs recognize the latter as the online idol Pop☆Step and begin mocking her. Koichi tries to stop them but gets beaten again.

Suddenly, Knuckleduster appears and defeats them with ease. Soga rises again and threatens to hurt Pop☆Step's face, but Koichi charges at him and knocks him down, allowing Pop☆Step to fly a safe distance away. Knuckleduster punches the thug leader into the wall which knocks him out.

After that, the thugs are offered samples of Trigger from Mario Kugutsu.

The next day, after seeing Koichi and Pop☆Step chasing Mario, Soga, Moyuru and Rapt, Soga uses the Trigger sample, turns into a Instant Villain, and attacks Koichi. But before he can do any harm, he is defeated and captured by Shota Aizawa, who is on his day off and just happens to be passing, while his teammates are also defeated and arrested.

Stendhal Arc
Days later, he is released, and with Moyuru, he visits Rapt who had been in hospital since the experience with Trigger. Rapt says that he enjoyed the experience offered by the drug, but when Soga asks Rapt if he would be interested in taking Trigger again, Rapt says no because although he liked that Trigger increased the capabilities of his Quirk, it is also the reason that he ended up hospitalized. He has already learned the lesson from his past experience and does not want it to be repeated.

Before this, Soga decides to take his friends away from the drug and is consequently forced to face Kuin Hachisuka, who had allied with Stendhal to get rid of the previous Instant Villains, including him and his friends. Kuin gives him the offer to inject Trigger to be able to fight against the murderous vigilante, but Soga rejects it, declaring that he will defeat him with his own abilities and without any kind of help. Faced with this refusal, Kuin frees a swarm of bees carrying Trigger and injects it to Soga by force, transforming him back into a Instant Villains to face Stendhal.

Koichi intervenes in the fight and tries to save Soga, understanding that Stendhal has no intention of simply defeating him and locking him up, just directly killing him. Koichi tries to escape, taking with him a badly wounded Soga, but Stendhal’s Quirk paralyzes them before he can. Just as he is about to finish them, Knuckleduster appears and defeats Stendhal, saving Soga and Koichi’s lives.

After being saved, he's hired by Knuckleduster to look into the whereabouts of his daughter and Kuin's host, Tamao Oguro, effectively making him Knuckleduster's informant. After Tamao is freed from Kuin's control, he and his friends watch over her bedside whenever Knuckleduster is out.