Kaylee Petrelli nee Ray was born in Cambridge, Massachusetts on a wintry February in 1987. Her mother Karen was a social service worker and her father Edward a physics professor at MIT. In her childhood, Kaylee was cared for and well-loved by her dedicated parents and thanks to their hard work there was little in her life that she wanted or needed. In the mid 1990s Edward was laid off from his job at MIT and struggled to find new employment. Supporting the family fell to Karen and eventually the family was forced to relocate to New York City when Karen found a better opportunity working at an adoption agency in lower Manhattan. Kaylee, a bright and personable young woman, quickly adapted to New York life. It was in New York where her father found work again, teaching physics at Columbia University.
Through her late teens, Kaylee took up dancing and pursued a degree in the arts at NYU. In 2006, Kaylee would gain the opportunity of a lifetime to dance in a small, off-Broadway performance of Giselle. It was here that Kaylee would catch the attention of Nathan Petrelli, an aspiring politician seeking to gain a seat in the New York State Senate. Nathan had just recently divorced from his first wife Heidi and found himself enamored by Kaylee's charm and charisma. Using his influence he managed to get backstage access and introduced himself to Kaylee, boldly asking her out to a dinner to which she famously said yes. Though Nathan lost his bid for the NY State Senate, he found something more in the relationship he forged with Kaylee. The two's public romance was widely covered by local tabloids like the Sheepshead Siren and some national media.
Nathan's public proposal to Kaylee in October of 2006 was overshadowed by media coverage of his younger brother's Peter's mental breakdown. Local media covered an argument between the Petrelli Brothers at the Orchid Lounge on the night of his proposal, famously ending with the brothers engaging in a fistfight on the street outside of the restaurant. Headlines showcasing the brothers' fight made national news for a brief time. Kaylee discreetly accepted Nathan's proposal the following day and the two were wed in a private ceremony in Martha's Vineyard in the spring of 2007.
By mid 2007 it became clear that Peter's mental health was in a dramatic decline. Peter checked himself in to Greystone Park Psychiatric Hospital at the urging of Kaylee and his brother and stayed there through most of the year.
In July of 2007 Kaylee and Nathan adopted their first child together, an eight-year-old special needs child named Brynn, though Kaylee was beginning to build a family of their own.
Just when Kaylee's life seemed to be on top, tragedy stuck in the fall of 2007. Peter escaped Greystone Park and came to the Petrelli Mansion to confront his brother in the middle of the night. Kaylee was awoken by the sounds of screaming and fighting, but by the time she awoke both Nathan and Peter were gone. Nathan did not return home that night, and the following morning police arrived to deliver the news that both Nathan and Peter had been found dead outside of Central Park in an apparent murder-suicide. According to the police reports, Nathan chased Peter to the rooftop of the Deveaux Building just south of Central Park, a residence that belonged to the late Charles Deveaux, a former hospice care client of Peter's. The police believe that Nathan and Peter had another altercation, ending with both falling from the roof. Forensics suggests that Peter tried to jump from the roof and Nathan attempted to stop him, but was not strong enough to support both of their weight and they fell more than thirty stories to their deaths.
In the wake of Nathan's and Peter's deaths, Kaylee was grief-stricken. Their mother Angela passed away two months later from a stroke, leaving Kaylee as the sole remaining heir to the Petrelli family fortune. Kaylee struggled at first to continue to raise her daughter, but found that Brynn's presence was an anchor that kept her going. She established a charitable organization, the Petrelli Foundation, in 2008 to combat the stigma of mental illness and raise awareness. In August of 2008 she adopted her second child, Jacelyn.
From 2008 to 2016 Kaylee endeavored to raise two young girls while managing a rapidly-growing charity. As the Petrelli Foundation grew, Kaylee's aspirations for growing charitable foundations only expanded, but she focused much of her time on motherhood, allowing for others to maintain the Petrelli Foundation in her stead. In 2016 Kaylee began to work her way back into the day-to-day operations of her charity, becoming a socialite in the public eye to make the necessary connections to secure donations for her foundation. In 2019 she purchased the small Lower East Side dance theater where she had performed Giselle and founded a nonprofit dance school for underprivileged children called the Cygnet.
By 2020, Kaylee Petrelli is a highly-regarded philanthropic presence in New York and an influential power among the city's wealthy elite.