At some point in her life, Cassia contracted a potentially fatal disease that disabled some of her senses. In desperation, Clove made a deal with Eggman to have her cyberized in exchange for their servitude. Cassia felt better afterward, but privately disliked being Eggman's slave. (StH: #279)
Cassia and Clove were charged with guarding the mining facility Crystal Cave, where a Chaos Emerald had been detected. While they were tending to business outside the mines, the Knothole Freedom Fighters destroyed the area's Badniks and managed to secure the Chaos Emerald inside. Seeing how easily their Badniks and E-1000 Series guards had been defeated, Cassia complains that she should have stayed behind to fight off the intruders. Clove expressed concern over the idea, worried about her sister's health, but Cassia assured her that she was fine thanks to her new cybernetics. Discovering the intruders to be Sally Acorn, Tails, and Antoine D'Coolette, Clove reported their findings to Eggman while Cassia grumbled about their boss's lousy mining equipment. They were then ordered to abandon the site and await reassignment.(StH: #263)
Later, Sally, this time accompanied by Bunnie Rabbot-D'Coolette, intruded into Cassia and Clove's territory again in search of a Gaia Temple and the cyan Chaos Emerald - the only emerald Eggman had managed to capture and retain. Before Clove left to investigate, Cassia asked to come along, but Clove wanted her to stay at the base where it was safer. To add to Cassia's dismay, the Egg Soldier on monitoring duty was busy playing video games and thus had not noticed the Freedom Fighters' incursion when it happened. Seeing an opportunity, Cassia decided to defy Clove's orders and went to the Chaos Emerald's shrine to investigate herself. Upon arrival, she found herself face-to-face with Bunnie. Refusing to lose another emerald, Cassia challenged Bunnie and the two cyborgs engaged in combat. Cassia was eventually overpowered after angering Bunnie with her taunts, but just then, the floor underneath them collapsed and they fell into a cave. Cassia hurt her ankle in the fall and Bunnie got the emerald. She blamed the rabbit for her misfortune, only to be rebuked, leading to a brief, personal argument. Bunnie, whose own experiences as a cyborg were just as personal, offered to help Cassia out of the cave, and Cassia reluctantly agreed. They eventually reached an storehouse bunker, where the Egg Army was waiting for them. Taking the emerald, Cassia decided to have Bunnie released for helping her, only for Bunnie to retake the emerald and flee. Despite this, Cassia ordered her troops to stand down, saying Bunnie wanted the emerald more than they did. However, she then fell ill and was carried back to base. While she was recovering, Clove learned from Eggman that Cassia's cybernetics did not, and could not, fully cure her illness, and that he lied to gain their loyalty. Regardless, without her cybernetics, Cassia's condition was fatal, leaving Clove with no other choice but to continue serving the mad doctor. Cassia apologized to her sister for her rash behavior, and expressed her wish to be rid of Eggman once her condition had been treated; Clove did not tell her what she had just learned to avoid hurting her feelings. (StH: #277, #278, #279)
When Clove was later called upon to join her fellow Egg Bosses for a special mission- retaking Eggmanland from Wendy and Walter Naugus and their forces- Cassia oversaw operations in her sister's absence. When Clove returned, Cassia set up a welcome-back party for her older sister. She and Clove later observed the rise of the Stone Stormlands Zone's Gaia Temple. (StH: #285(SU: #83, #86)
Compared to Clove, Cassia is vocal, hot-tempered, sarcastic, impulsive, and rather overconfident. While she understands and appreciates her sister's concern for her, she is also annoyed by it. Cassia also dislikes being under-equipped and weak; she blamed the purple Chaos Emerald's loss on shoddy mining and security equipment and felt jealous of Bunnie's arm-laser.
Despite her immaturity, Cassia does have a sense of honor and respect, as she let Bunnie go free as thanks for helping her out. Although she works for Eggman, she feels no true loyalty to him and only sees him as a necessary evil. She also looks down on Eggman's use of robots for infantry, as she does
Cassia is known to wield a pair of tonfa, and has enhanced abilities granted by her Cyberization.
She is apparently good at video games, with her high score being "Triple S".
- Artist Evan Stanley released concept art for Cassia which included a color scheme based on a Pronghorn. Stanley also included possible ideas for Cassia's back story, in which she underwent Cyberization due to a hypersensitivity condition that produced strong allergic reactions. Additionally, her tonfa weapons were described as being able to function as pepper-spray like dispensers in addition to melee weapons. 1 However, Ian Flynn confirmed that this back story is not considered canon. 
- Cassia appeared alongside several other villains on the variant cover for SU #73, despite not appearing in that issue or the story arc it was a part of. StH #275 had a variant cover that featured her and several other villains associated with Dr. Eggman, despite taking place in the Worlds Unite event.
- According to Aleah Baker, Cassia is about 13 years old.