Onjo was the founding monarch of Baekje

Baekje

Baekje or Paekche was a kingdom located in southwest Korea. It was one of the Three Kingdoms of Korea, together with Goguryeo and Silla....

, one of the Three Kingdoms of Korea

Three Kingdoms of Korea

The Three Kingdoms of Korea refer to the ancient Korean kingdoms of Goguryeo, Baekje and Silla, which dominated the Korean peninsula and parts of Manchuria for much of the 1st millennium...

. According to the Samguk Sagi

Samguk Sagi

Samguk Sagi is a historical record of the Three Kingdoms of Korea: Goguryeo, Baekje and Silla. The Samguk Sagi is written in Classical Chinese and its compilation was ordered by Goryeo's King Injong Samguk Sagi (History of the Three Kingdoms) is a historical record of the Three Kingdoms of...

, he was the ancestor of all Baekje kings.
He was the third son of King Dongmyeong (Jumong), the founder of the northern Korean kingdom Goguryeo

Goguryeo

Goguryeo or Koguryŏ was an ancient Korean kingdom located in present day northern and central parts of the Korean Peninsula, southern Manchuria, and southern Russian Maritime province....

. He was the younger half-brother of Yuri

Yuri of Goguryeo

King Yuri was the second ruler of Goguryeo, the northernmost of the Three Kingdoms of Korea. He was the eldest son of the kingdom's founder King Dongmyeongseong. As with many other early Korean rulers, the events of his life are known largely from the Samguk Sagi.- Background :Yuri was the son...

, who became Goguryeo's second king, and younger brother of Biryu

Biryu

Biryu was the second son of Jumong and So Seo-no, and older brother of Onjo, the traditionally recognized founder of Baekje , which was one of the Three Kingdoms of Korea...

, described in some records as the founder of Baekje.
Dongmyeong had three sons: Yuri

Yuri of Goguryeo

King Yuri was the second ruler of Goguryeo, the northernmost of the Three Kingdoms of Korea. He was the eldest son of the kingdom's founder King Dongmyeongseong. As with many other early Korean rulers, the events of his life are known largely from the Samguk Sagi.- Background :Yuri was the son...

, Biryu

Biryu

Biryu was the second son of Jumong and So Seo-no, and older brother of Onjo, the traditionally recognized founder of Baekje , which was one of the Three Kingdoms of Korea...

, and Onjo.