Yi Gu was born as the 4th son and 6th child of King Sejong and his wife, Queen Soheon.
From an early age, he was proud and kept a filial attitude to his parents and siblings.
[3] Like his second older brother, Grand Prince Suyang, he was proficient in martial arts, but he also created a scandal due to having sexual intercourse with court ladies and maids of the palace.
But this led to the Grand Prince to marry Lady Choi of the Jeonju Choi clan, later being titled as Princess Consort Jean, as his second wife and later marry Princess Consort Ahn of the Andong Ahn clan as his third wife.
[6] During the Gyeyujeong Rebellion, when Grand Prince Suyang fought for the throne against their nephew, King Danjong, Yi Gu supported and helped Grand Prince Suyang.