Always careful to walk at least several paces behind her baby-faced brother and keep out of shot if cameras are around, she looks so pale and fragile that its seems a strong wind might knock her down.

Indeed, compared to her obese and surly-looking sibling – North Korea's Supreme Leader Kim Jong-un – she seems a gentle soul, charming even, who couldn't hurt a fly.

When her brother met President Trump for a historic summit in 2019, she was seen shyly peering out from behind a wall. Observers thought it almost adorable.

And yet – according to a ground-breaking and revealing new book – those who judge Kim Yo-jong on appearances may be making a fatal mistake.

Believed to be 35, the youngest child of former North Korean dictator Kim Jong-Il is actually a ruthless political operator (even by her brutal family's standards) who some tip to succeed her brother and who their father regarded as the most able of his offspring.

Yo-jong may even be heading for an explosive power struggle with her niece – Jong-un's daughter Ju-ae – who is thought to be just 10 but has already been publicly hailed as her father's heir apparent.

But judging by what this new book reveals about Aunt Yo-jong with her 'trademark Mona Lisa smirk', it would be foolish to assume that little Ju-ae will one day be sitting on the throne of the Hermit Kingdom.

For Yo-jong, say North Korea experts, is 'the brains behind the operation' – and terrifying brains, at that.