Little is known about Cindy's life and past before the events of Jimmy Neutron: Boy Genius, except that she has a mom and a dad.
It is known that she moved to Retroville, Indiana and initially found herself the most respected and most intelligent person in her class at school, a position that she used to have. She also began a friendship with peers, such as Libby.
Jimmy took her "smartest person in Retroville" title when he moved into the house across the street from her whereupon Cindy, now second runner up, began a rivalry with him. She used to begin frequently attacks his intelligence by teasing him, playing pranks on him, and by criticizing his every mistake, even though most likely, she only does this to mask her love for Jimmy. She's frequently helped by Libby. Her home life is rarely shown, and her parents are only minor characters in the show. But some of the statements she makes during "Stranded" seems to suggest that she's not very happy at home.
Cindy is a member of the group of five, along with Libby, Carl, Sheen, and Jimmy, who usually go on his adventures with him. She began developing a deep friendship with these five, despite being the most aggressive and critical person in the group.
Penny Peterson, like Cindy, is also very rich and spoiled. She appeared to have been envious of Sherman's smarts and called him a dog and bullied him which resulted into a fight. She may also have a wicked sense of humor and spiteful as she would go far to get back at those she dislikes, as she bullied Sherman and in Egypt, when King Tut threatens to kill Mr. Peabody and Sherman, she says "You'd do that for me?", indicating she may have wanted that to happen out of spite for Sherman showing her up in class.
However, later they form a very strong friendship during their adventure. Sherman has a large crush on her. Penny also seems to have a crush on Sherman as well. So it happens that she and Sherman have a crush on each other. She also is very rebellious, as she tends to stray from Sherman and Mr. Peabody during the time traveling adventures, as shown when she wanted to stay in Ancient Egypt to rule with King Tut and keep his riches once he died, and when she took Da Vinci's flying machine for a test run with Sherman as an unwilling accomplice.
She is also manipulative and greedy, as shown when she wanted to marry Tut knowing he would die young in order to inherit his throne and riches (though eventually backed out once she realized when he died she would be killed and die with him) and when she tricked Sherman into showing her how Da Vinci's machine worked. Despite all of this, she is also shown to be willing to face up to her mistakes, as she proclaimed in front of everyone that it was her fault for provoking Sherman to bite her and that he didn't do anything wrong and attempts to take full responsibility for her actions in order for Mr. Peabody and Sherman to remain together (though this is not enough to convince the cold hearted Ms. Grunion to release Mr. Peabody) and apologizes for everything she has done. This proves not only can she understand her own mistakes, but is also loyal and caring to those she is close to.
But unbeknownst to the 2 girls, there's a secret hidden involving the 2, and one night will reveal it all.