A former North Korean diplomat speculates that North Korean leader Kim Jong-un has been keeping his sister Kim Yo-jong in check with frequent official appearances by his daughter.

Ko Young-hwan, who defected from the North as a diplomat and served as former vice president of South Korea's Institute for National Security Strategy, gave the assessment in an interview with Japan's Sankei Shimbun newspaper published on Tuesday.

Citing testimony from a former high-ranking North Korean official who also fled the regime, Ko noted that the emergence of Kim's daughter, Ju-ae, has coincided with photographs showing the leader’s sister physically distanced from the father and daughter.

The expert said that it was a deliberate move by state media exempt from coincidences.

Ko said that by putting his daughter in the limelight, Kim was trying to contain his sister's influence and have her focus on her original role of diplomacy mainly related to South Korea and the U.S.

The diplomat-turned-defector predicted, however, that one of Kim's two sons is more likely to be the heir as it is impossible for a woman to be the supreme leader in the North.

Unlike the Swiss-educated ruler, his three children are reportedly receiving private schooling in Pyongyang from top experts of various fields.