Kim Yo-Jong is the daughter of former North Korean leader Kim Jong-il and youngest sister of the current leader Kim Jong-Un.
As a direct descendant of the Mt Paektu bloodline, she was addressed by her parents as "Princess", and since her brother's ascention as leader has represented or been present with him at many high-level meetings with South Korea and the US.
Aged in her mid-30s, she holds the government rank of Vice Director of the Korean Workers Party Central Committee - yet as a new book explores, her power is much greater than that.
She controls the country's Propaganda and Agitation Department, and since 2021 is said to have gained authority to order executions.
Behind her back, North Korean officials reportedly call her "the Devil Woman" - with speculation she may one day take over as Supreme Leader.
Dr Sung-Yoon Lee is a North Korea expert based at Tufts University in the US and has explored Kim Yo-Jong in his new book The Sister.