Hayden first met Marner, then a forward for London of the Ontario Hockey League, when he toured the Children's Hospital at London Health Sciences Centre in 2015 with teammate and current Arizona Coyotes forward Christian Dvorak.
"Dvorak and I, that was kind of our thing," Marner said. "We'd go there and walk around. It's the OHL, but you still have the spotlight in London to be able to change someone's life. It was our goal just to make the kids feel better and forget where they were."
In Hayden's case, it meant making a new friend. And vice versa.
When Marner, who wears No. 16, joined the Maple Leafs, Hayden would watch him on TV. While in Toronto receiving treatments in the spring of 2017, she'd run up to the screen, point at Marner and yell out, "Go Mitch One-Six!"
"She still does that," Lindsay said.