"Lifelines" (WONDER WOMAN #105-108, 1996)
Cassie Sandsmark’s first appearance in DC Comics was in 1996's WONDER WOMAN #105. She was helping her mother Helena Sandsmark, and her mother’s new assistant, Diana (Wonder Woman in disguise), at the Gateway City Museum of Antiquities.
Being a young and curious teenager, Cassie stumbled upon an ancient warrior artifact and accidentally activated it. Unfortunately for Cassie, this sends the artifact into a killer rage. Wonder Woman suddenly appears to save the day, and with the help of Cassie, they are able to trick the attacking artifact and trap it at the bottom of a nearby lake.
"Level 3" (WONDER WOMAN #111, 1996)
In order to save her hero Wonder Woman from the murderous Doomsday, Cassie “borrowed” a few mystical items from the museum and joined the battle—the Sandals of Hermes and the Gauntlets of Atlas. The godlike powers of these artifacts proved to be too much for Cassie, and she was quickly overwhelmed. Still, Wonder Woman was impressed with Cassie’s spirit and recommended she be trained.
Cassie’s mother was not as impressed. She thought using the mystical items were too dangerous and forbid Cassie from using them. So, Cassie used them behind her back and took to the streets as Wonder Girl. Unfortunately for Cassie, a fight with Decay left her badly injured and, for her own good, Wonder Woman took away the Sandals of Hermes and the Gauntlets of Atlas.
The New 52 (TEEN TITANS, 2014-2016)
Cassie’s first appearance in the New 52 was in 2011's TEEN TITANS #1. She had just moved to Hollywood and was still enjoying her young life of crime. Unfortunately, her exploits had caught the attention of N.O.W.H.E.R.E. (a nefarious international organization with a more than questionable interest in metahumans).
A few days before Halloween, she was ambushed on the coastal highway by a fake policeman. Red Robin, who was tracking N.O.W.H.E.R.E., intervened. When he explained why he was there, Cassie was distrusting and reluctant. Not just because this metahuman-hunting organization now attacked a non-metahuman, but also because Red Robin was aware of her criminal activities. A renewed attack forced her to activate her armor. Though she defeated the attacking helicopter, she realized her life wouldn't be the same.
Cassie took Red Robin to her home, where he spent the night on the couch. He explained he was going to investigate other metahuman kids, but Cassie told him she was not interested in teaming up. She did decide to track Red Robin’s movements as he investigated Skitter, and saved him in an ambush by N.O.W.H.E.R.E. agent Thrice. She enjoyed her earlier investigation so much that she decided to join Red Robin and put her all into researching N.O.W.H.E.R.E.
Unfortunately, Cassie’s research had made her a target and N.O.W.H.E.R.E. ordered its prime agent Superboy to bring her in. On New Year's Eve, she was attacked in New York City. She managed to hold her own, and with the help of Red Robin’s new allies (Skitter, Bunker, Kid Flash and Solstice) they escaped. Superboy, after listening to stories about N.O.W.H.E.R.E. from both Solstice and Red Robin, doesn’t know who to believe, but pledges to never work for N.O.W.H.E.R.E. again.
Red Robin wanted to rescue Superboy from N.O.W.H.E.R.E., but the rest of the team was hesitant after their earlier fight. After a little convincing, Wonder Girl gets the team to join Red Robin’s crusade. After finding a way into the secret facility where Superboy was being held, Cassie soundly defeats Ravager and Templar. Unfortunately, after they freed Superboy and tried to escape, they were captured by the Super-Villain, Harvest.
"The Culling" (SUPERBOY #8-10, LEGION LOST #8-9, TEEN TITANS 2014-2016)
The evil Harvest had been gathering young metahumans to have them fight in an event called the Culling to thin the ranks of young power users. This time, the Titans were up. In the Crucible, they quickly teamed up with the Legion of Super-Heroes and the Ravagers. Red Robin devised a plan for them; the Legion would go after the central power core while the Titans held off Harvest's mobs. Wonder Girl worked together with Superboy, though she still had issues trusting him. When the Legion succeeded, Wonder Girl proposed escaping. Harvest would live, but his resources and base were destroyed, so he was out of the picture for now.
Cassie and Kon escaped together and landed on an island infested with dinosaurs. The two didn't get off to a good start, as their alone-time immediately resulted in bickering. As time progressed, they became more comfortable with each other. Cassie still didn't open up about her secrets, and considered all friendliness over after Superboy tried to kiss her. They eventually regrouped with the others, and Danny the Street brought them home.
Return of Diesel (TEEN TITANS, 2014-2016)
The more Cassie used her armor, the more difficulty she had controlling it. She suffered fits of rage, self-control issues and occasional black outs. Her repeated use of the armor eventually allowed Diesel, who had survived the temple collapse, to track her down. One night, everything went wrong. While the team was relaxing in their Lextowers base, Cassie lost control and the Silent Armor took over. Possessed by pure rage, she quickly defeats Bunker, Solstice, and Kid Flash. Superboy and Red Robin lure Cassie away from the city, and Superboy manages to remove Cassie's armor with his telekinesis. Diesel shows up after the battle and, before Cassie can recover it, absorbs the Silent Armor’s debris.
Determined to get the armor back, Cassie explains her history to her friends and leads them to Cambodia. Although Diesel showed he could control the armor, Cassie doubts that he would use the power for benevolent purposes. At the temple site, the team discovers the armor was somehow connected to the demon Trigon.
Suddenly, they’re attacked again by Diesel and a group of blood warriors. With no powers, Cassie leaves the warriors to her friends. She explains to Diesel she did everything because she loved him, and didn't want him to suffer. He didn't accept that, so Cassie uses the ace up her sleeve: her lariat was hidden the entire time. She whips it out and used it to ensnare Diesel. It drains the life from Diesel, and he withered away. Left with no host, the armor returns to Cassie.
Lost Friends (SUPERBOY #14, RED HOOD & THE OUTLAWS #16, TEEN TITANS 2014-2016)
While Red Robin was away on personal business, Cassie stepped up to become leader. But on her first outing, she could do nothing but watch as H'el took Superboy. So, when they discovered Red Robin was kidnapped by the Joker, she lead the team to Gotham City, desperate not to lose another member. They met up with Batgirl, who told them to leave as they weren't ready for Gotham. Wonder Girl decided to help either way. They teamed up with the Outlaws (Arsenal and Starfire) to distribute anti-Joker Venom vaccines to Jokerized mobs around Gotham. With that arduous task complete, she decided Batgirl was right, and waited for Red Robin to return on his own.
Trigon (TEEN TITANS, 2014-2016)
The interdimensional demon Trigon had set his eye on Earth, and sensed the presence of the Silent Armor. He planned to defeat the Titans once and for all by destroying their reputation and relationships. Trigon believed that if he used magic to possess Red Robin, he could destroy the Titans from the inside out. Red Robin began to showcase a slew of odd behavior, which even included seducing and sleeping with Wonder Girl. This "foreplay" heralded the arrival of Trigon on Times Square, which Cassie sensed through her connection with the Silent Armor. Wonder Girl charged into battle, but was quickly dispatched by Trigon. As he held her, he revealed that he knew a lot more about Cassie than she knew about herself, including who her true father was.
After the other Titans, Beast Boy, Psimon, and Raven join the fray, Cassie and Bunker defeat Ruskoff. Eventually, Red Robin managed to wound Trigon, which forcing the demon to retreat.
In the aftermath, Raven explains that Trigon tested Red Robin by lowering his inhibitions, and revealed some of the things - seducing both Cassie and Solstice (who was very much spoken for) - which he had done.
When Red Robin realized that Trigon's grasp was gone, he and the rest of team realize that all of his recent behavior actually originated from within and that he wasn’t brainwashed or possessed at all – and they were devastated.
Cassie was emotionally torn after learning about the catalyst for her encounter with Red Robin. She questioned her feelings for him - she enjoyed the night, and was in no way resisting his seduction. But she still really liked Superboy, a lot.
Lost in Time (TEEN TITANS, 2014-2016)
When the Justice League was defeated by the Crime Syndicate, the Titans were looking for their own way to defeat them. It didn't go well: Johnny Quick and Atomica defeated them, and Johnny sent them into the timestream. They bounced through several timelines before being sent to the near future timeline of Jon Lane Kent. She tried to find out if that timeline's League knew about her father, but a fight between Kon and Jon interrupted her search.
TITANS VERSUS THE ECHO CHRONAL AUTHORITY (TEEN TITANS, 2014-2016)
Gar Logan returned to Kid Flash's timeline and discovered he was now a wanted criminal. Superboy was recuperating in a Echo Chronal Authority hospital, with Cassie by his side. When he woke up, he attacked her in confusion, but she easily beat him. She didn't realize something the problem was far worse than confusion.
Kid Flash's trial ended up in an all-out war, with the Titans taking neither side. In the end, he pleaded guilty, and Solstice joined him in his imprisonment on Takron-Galtos. Superboy also left the Titans with in search of his own goals. With their numbers halved, Cassie wondered what would become of them. Red Robin confessed his love for her, revealing how he had bottled them up out of professionalism.
New Teen Titans (TEEN TITANS, 2014-2016)
Cassie, Red Robin and Raven were returned to their own time, where they reform the team with Bunker and Beast Boy. Skitter also returned. After a final confrontation with a not-quite reformed Harvest, they decided to continue the fight.
The Titans' return to New York. Their newly increased profile created two surprising consequences for Cassie. The good thing was, Cassie’s mother had finally decided to contact her again. The bad thing was that she had inspired a gang of young women to take up her name and appearance. Armed with crude weapons, these “Wonder Girls” were out on the streets dealing violent vigilante justice to robbers and other criminals who target women.