Vigilantes

Vigilantes are people who voluntarily carry out duties usually done by Pro Heroes without paying attention to the law.

Due to the hero regulations and Quirk restriction laws, people are not allowed to perform heroic duties out on their own and vigilantism is therefore illegal.

History
The origins of Heroes can be traced back to vigilantes. When Quirks started to manifest, there was no organization controlling the people who used their Quirks to help people and take down bad guys. Since these individuals were operating outside of the control of the government and law enforcement, they were declared vigilantes.

When the Pro Hero system was first put into place in Rhode Island, U.S.A. under the "Rhode Island New State Statute"; one hundred and eighty-nine vigilantes were affected by this statute, with only seven being accepted as official Pro Heroes. Nowadays most of the vigilantes have vanished.

However, Knuckleduster, Pop☆Step, and The Crawler had been regularly active in their activities to help people and dispense justice as vigilantes. Eraser Head stated that the Pro Heroes had never taken action against them and left those three be so long as they do not abuse their Quirks and go on a rampage.

Tenya Ida, Izuku Midoriya, and Shoto Todoroki technically acted as vigilantes during their battle with the Hero Killer: Stain.

Becoming a Vigilante
In the current age, there is not much reason to become a vigilante when one could simply apply for a high school education in Heroics and become a professional hero the legal way. Nonetheless, it does happen, despite vigilantism being illegal.

In some cases, it is a matter of principle, like how Chizome Akaguro dropped out of hero school because he was disillusioned by all of his classmates only caring about money and fame. He ended up becoming a vigilante and hunting down villains because he wanted to make up for the mistakes of hero society.

Other times, it can be a means to an end; Iwao Oguro became a vigilante after his hero career ended so that he could find his missing daughter and take down the group responsible for many of the misfortunes in his life.

Finally, sometimes a person can end a vigilante by a twist of fate; Koichi Haimawari had intended to apply into a hero academy, but missed his chance because he stopped to save a drowning child. Years later, he did good deeds around his neighborhood, which led to him saving a girl from a group of thugs. His innate bravery was noticed by Iwao, who chose to take Koichi under his wing.

Costumes
Vigilantes tend to wear costumes while they are out in the field. However, they don't have access to support companies like heroes do, nor do they resort to the black market like villains. Vigilantes basically just make a costume out of regular clothes.