Cynthia was a synthoid created by Louie for Howard Groote.
When Terry McGinnis came to the synthoid factory to purchase new synthoids for his training, he brought his friend Howard along. While Terry filled out the forms, Howard wandered about, meeting Louie, who was illegally creating synthoids that could pass as humans for customers. Desperate for a girlfriend to gain popularity, Howard requested a red-headed synthoid completely devoted to him.
The next day, the synthoid arrived at Hamilton Hill High School, earning the attention of many colleagues. Nelson Nash tried to hit on her but she told him she was only interested in Howard. When asked her name, Howard named her Cynthia. But Cynthia overheard Nelson's plan to "put [Howard] in his place." Cynthia pushed a row full of lockers on him, nearly killing him. When Howard's friends began talking about it, Cynthia said that Nelson wasn't a nice guy.
As he planned Howard's popularity began rising, so much that Chelsea Cunningham began flirting with him. However, Cynthia became over-possessive of Howard and prepared to drop a sign on Chelsea. Her plan had been foiled when Terry came to question her.
Later at Howard's party, Howard grew angry with the fact that Cynthia pushed and shoved anyone who showed the slightest interest in him. He decided to talk to Cynthia in his room (everyone thinks she's advancing on him). He tried to let her down easy but she overreacted and went ballistic. Batman arrived and tried to stop her but she proved to be too strong. Howard tried to appeal to her, promising they can be friends. At the last word, Cynthia overloaded then exploded, destroying Howard's house.
Although not designed as a battle synthiod, Cynthia possessed superhuman strength that surpassed even Batman's enhanced strength. She also possessed superman agility as she was able to twist her joints in ways that were impossible for humans.