Wonder Girl is a very tough young girl, with a very energetic, carefree and eager personality. In general, she prefers taking on foes stronger than herself. She loves to fight, and enjoys missions in which there will be physical confrontations. Sitting on the sidelines is less to her liking.
She can occasionally lose focus when impressed by something, but can regain focus when corrected. Her teammates have also remarked that stealth is not her strong suit.
Wonder Girl has a slender yet muscular build with blue eyes and blonde hair with unkempt bangs. Her hair is long, running down her back, and she wears a black headband. She wears a black sleeveless top with two stylized golden W's on it, which resemble the W's that Wonder Woman wears across her camisole. She has red pants with two white stripes on the sides, and blue shoes. When her Stealth-tech is active, her entire costume, shoes and all, turns black, with the exception of the stylized W's, which turn gray.
Like Wonder Woman, she wears two silver bracelets and star earrings (though silver instead of red), and uses a golden lasso that hangs at her waist.
On the way to their target, Wonder Girl explained she was excited about being on Alpha Team with him. Nightwing empathized, and told her of his early days on the Team. He also explained why he chose Cassie to come with: they were recruiting a new member, and as she was still new to the Team, she could be vital.
El Paso December 1, 08:19 MST
On the roof of the Rio Grande High School, Nightwing quickly went through their data on Blue Beetle with her, before sending her to meet Jaime Reyes, the Blue Beetle. Nightwing figured that sending a girl would reduce the chance of the meeting ending in a fight.
Cassie moved in and surprised Jaime, even more so because she explained she wanted to talk about Blue Beetle. Nightwing then joined the conversation, and offered their friendship.
El Paso December 1, 08:29 MST
Their conversation with Jaime was cut short when the Watchtower contacted Nightwing. While he conferenced with the Watchtower and the Cave, Wonder Girl kept Jaime from interfering.
El Paso December 1, 08:32 MST
Nightwing and Wonder Girl discussed what to do next. Cassie was especially upset that she wasn't abducted along with Wonder Woman, as Superboy did disappear. Jaime wanted to excuse himself, but Nightwing insisted that he'd consider. He told him that Ted Kord was a friend of his, and he would've wanted him to join.
El Paso December 1, 08:42 MST
Cassie tried convincing Jaime by telling him how the Team had helped develop her skills and powers, but Jaime was still hesitant. He wanted to discuss it with Peacemaker, Green Lantern and Captain Atom, but all were unavailable. Their conversation was cut short by another emergency, this time in Metropolis. Nightwing asked Jaime if he would like to come along.
New Mexico December 1, 09:00 MST
Alpha Squad took the Super-Cycle to Taos, with Blue Beetle in tow.
Alpha Squad arrived at Metropolis. After they landed, a number of Justice League members reported on their failure to penetrate the force field surrounding the city. Despite the bad news, Wonder Girl understood Blue Beetle's excitement at the events.
As Martian Manhunter and Flash attempted to communicate with the alien ship over Metropolis, Wonder Girl, Nightwing, Blue Beetle and Zatanna hovered nearby on the Super-Cycle, concealed inside a magic glamour bubble.
Metropolis December 1, post-17:54 EST
Alpha Squad successfully entered the Collector of Worlds's ship through a hatch that was opened to expel Martian Manhunter, Flash and Atom.
Metropolis December 1, 18:20 EST
Alpha made its way through the inner corridors of the vessel. Blue Beetle, still star-struck, engaged in a conversation with Zatanna. She revealed information about the Team and Nightwing to him, which led Cassie to conclude she had blown his mind.
Nightwing cut short the conversation, as he stumbled upon a room full of spheres. From a distance, Blue Beetle thought they were dioramas, but Nightwing discovered they were actually shrunk down cities.
Metropolis December 1, 18:31 EST
They continued on, planning to return for the shrunken cities later. But as they walked, Zatanna noticed her glamour was fading. Before she could put up a new one, the Collector of Worlds ambushed and stunned them.
Wonder Girl was still knocked out. Blue Beetle and Sphere managed to counterattack.
The Collector's Ship December 1, 19:01 EST
Alpha Squad regrouped and attacked the Collector, but the alien put up a forcefield. Just then, those who were on Kylstar's vessel were teleported in, as well as the Flash, Atom and Martian Manhunter, and they made quick work of the alien.
Metropolis December 1, 19:03 EST
The heroes and villains were cornered by dozens of new coludroids. Wonder Girl watched as Nightwing and Captain Atom persuaded the Collector to cease his attack.
Metropolis December 1, 22:03 EST
With Metropolis restored to full size, the heroes gathered in the park. Wonder Girl met up with Blue Beetle, and asked him if he had made up his mind. He had, but he had to convince his scarab he was going to join.
When Lobo was trying to kill secretary general Tseng, Wonder Girl and Batgirl were called in, and distracted the alien. Batgirl tried to escape with Tseng, while Wonder Girl took care of Lobo. Lobo eventually prevailed and revealed that Tseng was a robot, operated by a Krolotean. Lobo left with the Krolotean, as Wonder Girl looked on.
After the Justice League identified locations around the world with Krolotean activity, Wonder Girl was assigned as Alpha Squad along with Nightwing, to capture the remaining Kroloteans in Philadelphia.
Wonder Girl and her mentor, Wonder Woman, were part of Delta Squad during the mission to stop the Kroloteans on Malina Island. They were called in for backup when the enemies started to retreat. They both took out multiple Mechs. When they learned the base was set to explode, they picked up Aquaman and Lagoon Boy and retreated to the Bio-Ship.
As the Leaguers accused of crimes on Rimbor prepared to depart, Wonder Girl expressed her desire to come along. Wonder Woman refused, as only a select group would be necessary, and she could never convince her mother to allow it.
Aboard the Bio-Ship on her way to a mission to Bialya with Miss Martian, Bumblebee, and Batgirl, Wonder Girl was briefed by Nightwing, though she already knew what the mission details were. She laughed with the other girls after Batgirl called Nightwing out on why he had to justify choosing an all girl team.
Wonder Girl was upset when Miss Martian ordered her to stay behind as a lookout while the other three girls infiltrated the base. After protesting briefly, she accepted the instruction. Wonder Girl watched the three girls enter the base, before spotting a truck carrying Mammoth, Icicle Jr., Devastation, Shimmer, and Psimon, who was previously thought to be hospitalized. Realizing she couldn't contact her teammates without Psimon knowing they were there, she had to warn them by finding them in the base.
After finding Miss Martian, she set out to warn the others, but was discovered by Devastation, who pinned her down. She fought her and several Bialyan soldiers off, and made it to the rendezvous point. They were later joined by Bumblebee, who notified them of Batgirl's capture. They set out to liberate her, and the abductees she had found.
Wonder Girl stayed in reserve while the others rescued Batgirl. She forced open a gate where Batgirl could fly a plane with all the abductees through, and made sure Devastation couldn't get to her. Unfortunately, the plane was damaged, and without its wings, fell off the end cliff at the end of the runway. Wonder Girl tried to lift it up, but it was too heavy. However the Bio-Ship saved them both.
Cassie was disappointed their mission gave them so many unanswered questions, but Nightwing assured her that it was a success nonetheless. The other girls congratulated Wonder Girl on her successful mission with a group hug.
Cassie attended Raquel Ervin's bridal shower with all the other female members of the Justice League and the Team. After the party was interrupted by an armored car robbery by Captain Cold, Cassie assisted in his defeat.
After Alpha Squad infiltrated the Manta-Flyer in Star City, Nightwing, Miss Martian and Wonder Girl followed it discreetly in the Bio-Ship. She asked Nightwing how he knew about the new abduction and how he could know it would lead them to Lagoon Boy, to which he answered that Aquaman found the intel, and it was part wishful thinking.
They got close to their target, an underwater base. While Nightwing noted it was similar to the bombs, it reminded Cassie of Blue Beetle's armor. Miss Martian went in, and she and Alpha Squad liberated the captives. Wonder Girl stewarded the door as the Bio-Ship docked, and she oversaw the evacuation.
The operation was cut short by a Reach enforcer, who locked the bay doors. Wonder Girl threatened him and warned him he was outnumbered, and dubbed him "Black Beetle" in the process. The Black Beetle proved the superior warrior, though Cassie managed to land several futile punches before she was thrown aside. But she would not give up; she charged him with Superboy. It did not help. She contacted Nightwing for help, and squared off with the Black Beetle again. He pinned her down against the door, and mercilessly slammed her into it until she was down. Blue Beetle, Beast Boy and Impulse arrived, and while Blue Beetle held off the Black Beetle, Impulse helped Cassie aboard the Bio-Ship. There, she was fit enough to look after Superboy.
Wonder Girl joined other Team members in secretly boarding the Warworld, a giant satellite controlled by the alien despot Mongul. She was part of Alpha Squad with Superboy, Arsenal and Wolf, whose objective was to take down Mongul. Alpha Squad first had to battle drones that protected Warworld before confronting Mongul. The alien despot proved too powerful for them to take down, but after Bumblebee created a power surge to stun Mongul, Alpha Squad was able to subdue him.
The Warworld May 26, post-12:40 UTC
Alpha Squad joined the rest of the team in the docking bay, having taken longer to get there because of how heavy Mongul was. As Wonder Girl and Superboy carried Mongul, Arsenal watched as Bumblebee and Guardian shared a kiss and wondered how he might get one. Wonder Girl then gave Arsenal a peck on the cheek. Blue Beetle then suddenly hit Impulse with the crystal key he had retrieved and attack the rest of the Team. Superboy was knocked aside and Wonder Girl was trapped underneath Mongul and pinned down by Beetle with a staple. After Arsenal panicked and opened the airlock, Beetle was able to close it and take down the rest of the Team, except for Arsenal, who escaped.
The Warworld May 30, post-03:36 UTC
Wonder Girl and the rest of the Team were held in stasis chambers as prisoners of the Reach. Arsenal, along with four teens that the Team had rescued from the Reach's underwater base, ultimately freed the captured Team members.
While Zatanna was preparing a ritual to prevent the Reach from controlling Blue Beetle, Green Beetle arrived to try and free him. Batgirl and Impulse tried to stop him, but did not succeed and were knocked out. Luckily, Wonder Girl, Robin, Bumblebee, and Beast Boy intervened. Wonder Girl lassoed him, while Robin attacked him with a flying kick. They managed to keep him preoccupied until Zatanna completed the incantation, and returned control of their bodies to Green Beetle and Blue Beetle.
When magnetic field disruptors planted by the Reach threatened the Earth, Wonder Girl joined 39 other heroes of Earth in a briefing in Metropolis City. The heroes would use a computer virus provided by Lex Luthor to safely disable the disruptors. The heroes were paired off with another into twenty squads: one member would run interference against weapons guarding the disruptors, allowing the other to get close enough to plant the virus. When Wonder Girl expressed her incredulity at their working with Lex Luthor, Robin told her it wasn't in Luthor's best interest for Earth to be destroyed; at any rate, they don't have any other alternative.
Wonder Girl was assigned to Tau Squad, and worked alongside Robin. Within thirty minutes, all squads had succeeded in their mission.
Tim and Cassie welcomed Static to the Team, and inquired about the whereabouts of his former teammates. Unbeknownst to Static but noticed by Superboy, the pair were holding hands behind their backs. After Wally's death, they had become a couple after she gained enough nerve to kiss him. Aqualad summoned everyone to the mission room, where Batman informed the Team that they would now operate side-by-side with the Justice League from the Watchtower. Not because Mount Justice was destroyed, but because they earned the right to do so. Aqualad assigned Wonder Girl to Gamma Squad to investigate Batgirl's lead about the whereabouts of Vandal Savage.
This Wonder Girl is based on Cassie Sandsmark; Cassie is the third person to be called Wonder Girl. The first was Diana (the later Wonder Woman), the second Donna Troy, a founding member of the Teen Titans. Cassie is the daughter of Zeus and archaeologist Helena Sandsmark, a friend of Diana's. She first became Wonder Girl clandestinely (and sporting a black wig), and was only later accepted by Wonder Woman. She joined Young Justice, and later became the leader of the Teen Titans. Following the 2011 reboot, Cassie's history has been altered, removing her connection to Wonder Woman.
This is Cassie's first animated appearance.
Wonder Girl was one of the few characters declared off-limits in the early production process, meaning that she could not be used during the first season. That restriction was eventually lifted.