Founding ancestor of the Deoksu Yi clan; military official of Goryeo (Jungnangjang of the Sinhowi guard)
| Era | 고려 |
| Clan | 덕수 이씨 |
Yi Don-su (李敦守) is the founding ancestor of the Deoksu Yi clan and a military official who served as Jungnangjang (中郞將, mid-ranking general) of the Sinhowi guard during the reign of King Gojong of Goryeo. His birth and death years are not clearly recorded, but are estimated to be roughly from 1177 (7th year of King Myeongjong) to around 1240 (27th year of King Gojong). Admiral Yi Sun-sin (Chungmu-gong) and Yi I (Yulgok) were his descendants.
According to the Biography of Jo Chung (趙沖傳) in the Goryeosa, when Khitan refugees (the Later Liao state) crossed the Yalu River and invaded Goryeo from 1216 to 1219, Yi Don-su was active as a general. In 1218, while the Goryeo army led by Commander Jo Chung was awaiting reinforcements, the unit of Yi Jeok (李勣), the Gyeongsang Province inspector, was blocked by the enemy; Yi Don-su and Kim Gye-bong were dispatched, defeated the enemy, and safely reunited Yi Jeok's unit with the main force. The Goryeo army then isolated the Khitan refugees in Gangdong Fortress (江東城), and in 1219 the allied Goryeo–Mongol forces completely defeated them.
Although members of the Yi family had lived in Deoksu-hyeon (德水縣) since the Goryeo period, their earlier lineage was unclear. When the fourth-generation Yi Yun-un (李允蒀) served as Pandopanseo and was posthumously enfeoffed as Deoksu Buwongun, his descendants took Deoksu as their ancestral seat. The lineage was established based on the family genealogy of Yi I (Yulgok), which took the Jungnangjang Yi Don-su as founding ancestor; it was later organized by Yi Sik (Taekdang) and firmly established with the publication of the first Deoksu Yi clan genealogy in 1713.
His descendants include Admiral Yi Sun-sin (Chungmu-gong) and Yi I (Yulgok).