Cassandra "Cassie" Sandsmark is the daughter of archaeologist, Helena Sandsmark. Helena is an old friend and former student of Julia Kapatelis, Wonder Woman's mentor in Patriach's World. Cassie spent most of her time at her mother's work at the Gateway City Museum of Antiquities, where she had a penchant for getting into trouble. She quickly befriended her mother's new employee, Wonder Woman, and was eager to help her. During a fight with a Doomsday clone and another battle with Decay, she created a costume and used magical acquirements (the Sandals of Hermes and the Gauntlet of Atlas) to help Wonder Woman, much to her mother's horror. Cassie later had the opportunity to ask Zeus for a boon, and requested real superpowers. Zeus granted her request, but gave Dr. Sandsmark the ability to deactivate them. Dr. Sandsmark, however, reluctantly accepted her daughter's wish to be a superheroine and rarely if ever uses this ability. Artemis was assigned as her teacher.
Cassie heavily idolized Donna Troy, the original Wonder Girl, and to that effect wore a black wig over her natural blond hair (it also helped to conceal her identity). Donna gave Cassie the second costume she wore as Wonder Girl, but Cassie was afraid to wear it lest it were ruined (she did don the outfit during the "Sins of Youth" crossover when she was temporarily aged to adulthood). As Wonder Girl, Cassie joined Young Justice due to her crush on Superboy. She became close friends with Arrowette, Secret, and Empress. During her time with that group, the dark god Oblivion caused Wonder Woman's Wonder Dome palace to destroy Gateway City. Artemis ordered Cassie to help with crowd control instead of wasting time putting on her Wonder Girl disguise. Agreeing with her, Cassie revealed her secret identity before news cameras for the first time in her superhero career.
When that group disbanded, she joined the new Teen Titans to learn more about being a hero, and at around this time she also discovered that Zeus was her true father. The Teen Titans became her closest friends and she grew particularly close to Superboy, forming a romantic relationship with him that was cut brutally short by his death in the 'Infinite Crisis'. In the year that followed, her path took her away from the Teen Titans, who she felt had let her down, and got her involved with some very strange people. However, she eventually rejoined the Teen Titans and was overjoyed when Superboy returned from the dead.
- Wonder Girl (Vendor)
- Wonder Girl (New Troy Central Station)
- Wonder Girl (Gotham Under Siege)
- Wonder Girl (Legends PvP)
- Wonder Girl can be found with other magic users socializing in the Oblivion Bar.
- Wonder Girl is a playable character for Legends PvP and Legends PvE.
- Wonder Girl is captured by Giganta and Circe for a ritual of mind transference to permit Giganta an Amazonian body. She absorbs Wonder Girl's power, then attacks the player.
- Wonder Girl can be found as a Vendor in the Watchtower Magic Wing.
- She is the final boss in the New Troy Central Station instance.
- Wonder Girl is patrolling the Gotham Wastelands with the Titans.
- In Trigon's Prison, Wonder Girl may assist in combating distressed spirits in the upper levels of the catacombs.
- Wonder Girl is involved in the No Small Wonder story arc where she is captured by Giganta and Circe for a ritual of mind transference to permit Giganta an Amazonian body. Giganta becomes greedy and tampers with the ritual, transferring her powers to Wonder Girl rather than the other way around. It's up to newly recruited exobyte members of the Society of Super-Villains to reverse the spell and defeat Wonder Girl so that Giganta can escape!
- She is the final boss in the New Troy Central Station instance for New Villains.
- Starfire, Beast Boy, Superboy, Wonder Girl and Cyborg will appear to assist Raven's Good Soul Projection in the Gotham Wastelands if it comes under attack from New Villains.
- She is an iconic enemy for villains in Gotham Under Siege.
- Wonder Girl's Combat Bracers
- Wonder Girl's Waistband
- Wonder Girl first appeared in Wonder Woman #105 (February 1996)
- Wonder Girl is voiced by Mindy Raymond
- Wonder Girl's Lasso of Lightning, which can electrify her enemies when triggered by her anger, was a gift from her half-brother, Ares. The lasso's energy stems directly from the lighting of Zeus.
- Cassandra is locked in a tepid rivalry with Vanessa Kapatelis. Vanessa and her mother, Julia Kapatelis, allowed Wonder Woman to live with them in Boston during the hero's first five years living away from Themyscira. As much a fan of Wonder Woman as Cassandra was, Vanessa harbors resentment over how Cassandra got to become Wonder Girl while she did not. This resentment was warped by Circe and Doctor Psycho, who turned Vanessa into the second Silver Swan and sent her out to kill Wonder Woman's friends as vengeance for "abandoning" her.
- While their daughters are best of friends, Cassandra's mother, Helena Sandsmark, and Arrowette's mother, Bonnie Jones, despise each other due to disagreeing over each other's methods as mothers (Bonnie believes Helena is too restrictive and negative on her daughter's choice to be a hero, while Helena believes Bonnie is forcing her daughter to live the life that she wanted to live).