William's incredible gesture: Canberra boy to climb Mount Kosciuszko in memory of his sister

What distinguishes him from most other children his age is a keen sense of empathy and an understanding that sometimes you have to do something difficult to help others.

So, it was without hesitation the Year 1 student from Evatt signed up to walk to the top of Australia in memory of his late big sister Annabelle, and to raise funds and awareness for Rare Cancers Australia.

On March 19, William will be walking a 13km round-trip from Eagles Nest to the summit of Mount Kosciuszko, every step a reminder of his love for Annabelle, who died aged five in 2019 from an aggressive brain tumour.

Why does he want to scale 2228 metres above sea level? It's really very simple.

"Because I want to raise money for people who have sick kids, like with brain cancer," William said.

All of the funds raised in the Kosi Challenge go to Rare Cancers Australia, which provides practical support to the more than 52,000 Australians diagnosed with a rare and less common cancer each year.

William was only three when Annabelle died but he still remembers her kind nature.

"She was always nice to me. She was only mean to me the one time," William said, making their mum Kathie laugh.

"She was a good sister. She would look after me when I was sick."

Kathie and husband Adam will also be doing the walk, carrying their daughter Vivianna, 2, on their back.

Their other daughter Juliette, 3, will be cheering from base camp with friends and family.

Annabelle was diagnosed with diffuse intrinsic pontine glioma, (DIPG) a brain tumour that is highly aggressive and difficult to treat.

Her parents tried everything they could, from clinical trials in the United Kingdom to chemotherapy in Mexico.

Kathie said Rare Cancers Australia helped them, as it helped many families in Canberra, as there is no treatment options in the national capital for children with cancer, meaning all families have to travel for medical attention.

Rare Cancers Australia assisting with everything from researching treatment options to helping with travel costs - and providing a ready-made community who understood what the family was going through.

"You feel really isolated when you're dealing with a rare or less common cancer," Kathie said.

William has been training on Mount Rogers, Mount Ainslie and One Tree Hill.

He and Kathie remember getting caught in the rain on One Tree Hill, something they didn't expect, because on their walks home from school, Annabelle always seemed to protect them from the rain.

Then they realised, in the distance, there was a fire.

Annabelle was helping to put out the fire, still looking out for them.

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