While the hero who runs the agency is the primary field operative who rescues people and fights villains, the sidekicks serve as backup for their employer. The number of Sidekicks at an agency can vary; some Heroes have only a couple of sidekicks, while others have dozens, and some Pros opt to work without any sidekicks.
As with any business, the employer has the freedom to hire or disregard any potential sidekicks they like. The employer hero will generally choose to hire sidekicks who they think could be useful, one way or another. Some heroes will elect to hire sidekicks based on their skills and the potential uses for their Quirks. Other heroes, such as Uwabami, hire sidekicks solely for their good looks, so as to help with their own popularity in the celebrity aspect of being a hero.
Some heroes require their sidekicks to wear a certain type of uniform, usually using their own costume as base. For example, Best Jeanist has all of his sidekicks wear a smart pair of denim jeans, and has them keep their hair neat and styled. Not every agency has a dress code though.
Sidekicks make an effort to get hired by more prestigious Pro Heroes, whether out of admiration or to gain popularity and experience in order to eventually become full-fledged Pro Heroes themselves and open their own agencies. Though, most sidekicks usually stay at their rank and don't continue on to become Pros themselves. There are some exceptions to this standard, however. Some sidekicks, such as Sir Nighteye, have gone on to start their own Hero Agencies with their own sidekicks working under them.
After passing the Provisional Hero License Exam and getting a Provisional Hero License, hero students are allowed to use their Quirks in emergency situations, preferably under the guidance of a fully licensed hero, and are thus allowed to undergo part-time employment at a hero agency that is willing to hire them.
In some cases, such as the aftermath of the U.A. Sports Festival, Heroes actually send recruitment requests to the students whom they wish to have work under them. Other times, a student simply undergoes an ordinary job interview in order to become an intern. In this case, it would help to have the introduction and recommendation of someone who already works there, even if it's another intern, like when Mirio introduced Izuku to Sir Nighteye.
During this work-study, the students are treated like real sidekicks, and are allowed to participate in relief efforts during emergency situations. This allows them to gain real experience and can help them get noticed by other Pro Heroes who might want to hire them as full time sidekicks after they graduate, assuming they don't end up becoming a full time employee at the agency where they are interning.
- In traditional superhero fiction, a hero would normally only have a single sidekick: a young protégé with the same powers and/or skill set. They would usually not take in a new sidekick until the previous one graduated into a full fledged hero, or was no longer able to continue the fight against evil.