Aqualad, who had taken charge during the Cadmus incident, acted as the leader of the Team, though other members of the Team have acted as leader in situations where not all members are available for a mission. As time passed, the ranks grew as more members of the Justice League introduced their young protégés or other young heroes to the Team. Four years later, Nightwing became the new leader of the Team. Aqualad resumed leadership of the Team after his mission to infiltrate the Light ended.
Prior to the formation of the Team, Robin was the first junior protégé to begin his crime fighting career, starting in 2006, at the age of nine.A year later, Green Arrow introduced Speedy as his partner.
Inspired by his Justice League colleagues, Aquaman offered both Garth and Kaldur'ahm a position to become his protégé after they saved him from Ocean-Master. Kaldur became Aqualad.Flash was not fond of the idea of the responsibility of a sidekick, but his nephew Wally differed. He recreated the accident that gave his uncle his super speed power. Partly out of guilt, Flash let him become his protégé, naming him Kid Flash.
Before the Cadmus incident, the four sidekicks knew each other; and Robin entrusted Kid Flash with his secret identity. However, the four of them had never been together all at the same time.
Last to take a protégé was Martian Manhunter. Miss Martian claimed she had won the right to become her uncle's sidekick after winning a competition on Mars, though this may not entirely be accurate, as Martain Manhunter has claimed he found M'gann stowed aboard his ship, and claimed to be his niece.
The four sidekicks were brought to the Hall of Justice as a graduation. However, it was nothing but a "glorified backstage pass", as Speedy called it. Fed up with being lied to, Speedy left. As the League members moved to the Watchtower, the others investigated a minor case: a fire at Project Cadmus. This led to the discovery of Project Kr and Superboy. Cadmus's chief scientist, Mark Desmond, became Blockbuster in the hopes of destroying the intruders, but was defeated.
The Justice League arrived at the scene of the destroyed Cadmus facility, and Batman was not happy. He did not, however, expect the young heroes to rebel against the League. They insisted they did good work, and wanted to work together more. Batman asked for three days to plan things.
Batman gathered the four in the old Justice League hideout inside Mount Justice. They would become a team, on Justice League's terms, handling covert ops missions. They were introduced to the fifth member of the Team, Miss Martian.
Kid Flash was eager to let Speedy join, but he declined, claiming their "junior Justice League" was a joke. He was not interested in fighting on the Justice League's terms, and claimed he was tired of them telling him what to do.
During a solo mission, Red Arrow and Aqualad learned from Sportsmaster that there was a mole on the Team feeding information to their enemies. Both would investigate. Aqualad offered his friend membership, but he declined; he did change his opinion about the Team.
Red Arrow had been elected into the Justice League, and to set the right example, Green Arrow told him to join the Team before being inducted. This did not sit well with Artemis; Red Arrow, in the meantime, considered it a good opportunity to investigate who was the mole. He eventually concluded there was none, and Sportsmaster deceived them.
The Light tried to coerce three members—Superboy, Miss Martian and Artemis—into joining them. They had already shared their secrets with the rest of the Team, and successfully ambushed their opponents.
They started training a number of young heroes; thirteen members joined in four years time. This new format required organization, and the Team would be split up into smaller groups—Alpha Squad, Beta Squad, Gamma Squad, Delta Squad, and occasionally others—like Zeta—which would be supervised from the Cave by Mal Duncan.
The Team succeeded in rescuing the four captive members, as well as a number of abducted teens, from a Reach ship. In the process, Miss Martian psychically attacked Kaldur's mind in revenge for Artemis' "murder", discovering too late that it was all a ruse.
When the Hall of Justice was sealed off by Despero, Mal donned Jim Harper's old Guardian costume to help fight the alien warrior. He would keep the identity and officially join the Team. Though Despero and his major domo L-Ron were defeated, the Hall of Justice was destroyed.
Blue Beetle attacked the rest of the Team, leading to the capture of Superboy, Robin, Batgirl, Guardian, Bumblebee, Beast Boy, Impulse, Wonder Girl and Wolf. Arsenal was able to escape, thanks to recklessly venting the atmosphere. Sphere was blown into space.
While searching for the Team, Nightwing and Miss Martian were reunited with Sphere. Arsenal and four superpowered, former Reach captives rescued the captive Team members. Once the two parties met and retreated, Nightwing offered the runaways membership of the Team. They ultimately rejected the offer after Nightwing expelled Arsenal from the Team for endangering lives.
Artemis had decided to return to the Team as Tigress. Impulse became the new Kid Flash. Virgil Hawkins joined the Team as Static. Nightwing told Aqualad he would be taking a leave of absence from the Team; Batgirl would step up in his place. Batman informed the Team they would now operate from the Watchtower, side-by-side with the League.
Superboy (Conner Kent): A genetic hybrid clone of Superman and Lex Luthor. Throughout his first year, Superboy struggled to come to terms with his hidden aspirations to gain the respect and love of the Man of Steel himself, whom he saw as his father.
Superboy (Conner Kent): One of the senior members of the Team. He has been offered membership of the Justice League, but has turned it down. He still has an open invitation to join the League.
Tigress (Artemis Crock): Part of the original team, she quit the Team to attend college with Kid Flash. She later returned only to fake her death and start working undercover on a new mission as Tigress. After Wally died saving the world, she decided to join the Team again, as Tigress.
Arsenal (Roy Harper): Joined shortly after his release from captivity. He was dismissed for endangering the lives of his teammates for his own personal goals, and his lack of teamwork skills. He was later invited to return, but declined, preferring to stay solo.
Kid Flash (Wally West): one of the founding members, he quit the hero life and left the Team. He came out of temporary retirement to help Aqualad and Artemis finish their deep cover mission, and stop the Reach invasion. He died saving the world.
Nightwing (Dick Grayson): The original Robin and former second-in-command of the Team. He has been offered membership of the Justice League, but has turned it down. He still has an open invitation to join the League. Nightwing was the leader of the Team for a year, while Aqualad was on a deep-cover mission, and stepped down when Aqualad returned. Following the death of Wally West, Nightwing decided to leave the team citing "a leave of absence".
Batman: Batman is the supervisor of the Team and is responsible for briefing and deploying them on covert missions.
Red Tornado: Red Tornado volunteered to stay at Mount Justice to watch over the Team as sort of "den-mother", or rather den robot. He has maintained that he will not interfere in the middle of a Team mission for support. After an attack on the Cave by his "siblings", Red Torpedo and Red Inferno, Red Tornado temporarily left the Team to pursue his creator, but returned after they found him, claiming he has grown fond of the Team.
Black Canary: Black Canary works as the Team's hand-to-hand combat instructor and trainer. She also acted as a third replacement "den-mother" for a week. On one occasion she even took the role of counselor, after a disastrous psychic training exercise took place in which all six teenaged members of the Team were killed in action.
Each member of the Team, as well as Red Arrow, has a membership number, which is read out when they use the Zeta-tubes to enter the Cave located in Mount Justice. The first three designation numbers indicate the order in which the individual became active as a hero/protege. B04 and B05 indicate the order of introduction to the other founders. Speedy, later Red Arrow, was given the designation of B06 to indicate his open invitation, which he eventually, though briefly, accepted. B07 and onward indicate the order in which they joined the Team.
The following designation numbers of the Team are known:
↑Although known to fans as "Young Justice" (based on the name of the series and by comparison to the comic book team after whom the series is named) this term has never been used within the series itself. This team is referred to purely as "The Team". Greg Weisman has confirmed that the group's official name is "The Team", since it is not a public team and doesn't require a "fancy name" (source). The blurb from the DVD releases however refer to the Team as Young Justice.
↑Though Sergeant Marvel's identity has yet to be explicitly confirmed, every other Team member's secret identity is known and Greg Weisman has indicated Mary Bromfield was a member of the Team. Thus, Mary must be Sergeant Marvel.