A young Ontario girl who is featured prominently in the Hospital for Sick Children's multi-million dollar "VS." ad campaign has died, her mother said in a private Facebook post this week.

"Fly high and free my precious girl. After nine and a half years of fighting, you certainly deserve those wings," wrote Christie Joy Chiasson about her daughter, Jadyn "Super-Girl" Schill.

Jadyn passed on March 6, according to the post. She was 15 years old.

In 2013, SickKids dubbed Jadyn the most radiated child in Canada, having received a total of 123 doses of radiation as part of her treatment for cancer. That's the maximum anyone is allowed for life.

When she was just five years old, Jadyn was diagnosed with a form of cancer called childhood Ependymoma, which manifests itself in the brains of children. In adults, it most commonly occurs in spinal tissue.

Jadyn Schill, 13, has had brain cancer since she was five. She updates CBC Toronto on her condition. 3:38

CBC Toronto first met Jadyn in the summer of 2016, when she was fighting a seventh recurrence of the disease. At the time, she was the first child in the world to receive an experimental treatment that ultimately failed to stop the cancer from propagating further.

During her years battling the illness, Jadyn became a patient ambassador at SickKids, participating in fundraising events and even forming friendships with celebrity backers of cancer research, including Billy Baldwin and Kim Coates, an actor on the hit series Sons of Anarchy.

Her optimism and fierce determination to press on in the face of daunting odds made her somewhat of a celebrity herself in the halls at SickKids.

Jadyn and her mother Christie photographed at Sick Kids prior to a surgery in January 2013. (Christie Schill)

"Surround yourself with what you love," Jadyn told CBC Toronto from her home in Mount Albert, Ont., in 2016, her room laden with posters, Maple Leaf jerseys and a custom-made 'Super-Girl' cape hanging on the wall.

"Keep the positive attitude and you're good."

Around SickKids, Jadyn Schill is known as Super Girl. With good reason. The 13-year-old from Mount Albert was first diagnosed with a rare form of brain cancer when she was 5. She's now battled the disease seven times. The CBC's Shannon Martin tells her story. 3:28

That same year, Jadyn became one of the faces of SickKids much-lauded and highly memorable $2-million advertising campaign called "VS." In television commercials and social media ads, on billboards throughout Toronto and plastered onto TTC streetcars, real SickKids patients were featured fiercely staring down representations of various illnesses like cancer, cystic fibrosis and heart disease.

While the campaign garnered some criticism for how it chose to invoke concepts of warriors and fighters — a framing that some argue is not a productive way to talk about childhood illness — it was largely celebrated as an ambitious departure from typical health care fundraising initiatives. It turned out to be so successful that SickKids used it as a launching pad for its newest effort to raise $1.3 billion to building a new, state of the art facility.

In an interview, SickKids Foundation associate director Lisa Charendoff said that Jadyn's death has been felt deeply by those who knew her and her good works.

"She was a really extraordinary, I would say indefatigable, ambassador for SickKids and for brain tumour awareness," she told CBC Toronto.

"There was never an opportunity that she would turn down to share her story and make sure people knew more about brain tumours and pediatric brain tumours."

Jadyn and Kim Coates, an actor who starred in the show Sons of Anarchy, became good friends after meeting at the Car Rally for Sick Kids several years ago. (Christie Schill)

Charendoff said that she herself is always hesitant to use language like "warrior" and "fighter" to talk about children, she admits that in Jadyn's case, it is apt.

"She had a real spirit about her. She fought to raise awareness, and she fought to raise funds. And she fought for every bit of time she could have during which she could live well.

"We know that her legacy will carry on because she's touched so many people."