Ji You 季友 "Youngest Brother You" (died 644 BCE), also called "Ducal Son" Gongsun You 公子友, Gongsun Ji You, 公子季友 or Cheng Ji 成季 "Perfected Younger Brother", or Jizi 季子 "Viscount Youngest Brother", was a son of Duke Huan of Lu 魯桓公 (r. 712-694) and the ancestor of the lateral house of Jisun 季孫. He served the Dukes Min 魯閔公 (also written 魯湣公, r. 662-660) and Xi 魯僖公 (also written 魯禧公, 660-627) of the state of Lu 魯 as a high minister during the Spring and Autumn period 春秋 (770-5th cent. BCE).
Duke Huan had several other sons, the most important of which were Prince Tong 同 (the eventual Duke Zhuang 魯莊公, r. 694-662), Shu Ya 叔牙, founder of the house of Shusun 叔孫, and Qing Fu 慶父, founder of the house of Zhongsun 仲孫. The latter was the son of a secondary wife.
When Ji You's older brother, Duke Zhuang, fell ill, he consulted his brother Shu Ya about a possible successor, and Shu Ya recommended their common brother Qing Fu, while Ji You preferred the crown prince Ban 般 (also written Ban 班). Duke Zhuang, doubting Shu Ya's loyalty to his brother, ordered Ji You to kill Shu Ya.
When the Duke died, Ji You enthroned Duke Zhuang's son Prince Ban. Qing Fu killed Ban and enthroned another son of Duke Zhuang, Prince Qi 啟, who is known as Duke Min, but in 660 planned to mount the throne of Lu himself. His attempt to have Duke Min assassinated failed and he fled to the statelet of Ju 莒.
Ji You enthroned Prince Shen 申, who is known as Duke Xi. He also bribed the Lord of Ju to hand over Qing Fu, whereupon the latter committed suicide. Because the Lord of Ju had taken the bribe but did not hand over the corpse of Qing Fu, Ji You had attacked Ju. In the battle of Li 酈, Ji You captured the younger brother of the Lord of Ju, Ju Ru 莒挐. Duke Xi rewarded Ji You with the territories of Wenyang 汶陽 and Fei 費.
His descendants were known as the house of Jisun 季孫, literally "grandsons of the Youngest Brother". The Jisun family became one of the mighty Three Huan families 三桓 that challenged the ducal house of Lu.
|Duke Huan of Lu 魯桓公 (r. 712-694)|
|Gongzi You (Ji You 季友, Jizi, Cheng Ji 成季, Chengji You 成季友)|
|Qizhong Wuyi 齊仲無佚|
|Ji Wenzi 季文子 (Jisun Xingfu 季孫行父)|
|Ji Wuzi 季武子 (Jisun Su 季孫宿)|
|Gong Mi 公彌 (Gong Chu 公鉏)||Ji Daozi 季悼子 (Jisun He 季孫紇)||Jigong Niao 季公鳥||Jigong Ruo 季公若 (Jigong Hai 季公亥)|
|NN||Ji Pingzi 季平子 (Jisun Yiru 季孫意如)||Gongfu 公甫||Gongzhi 公之 (Jian 堅)||Gongfu Mubo 公父穆伯 (Jisun Jing 季孫靖)||Shen 申|
|NN||Ji Huanzi 季桓子 (Jisun Si 季孫斯)||Ji Wu 季寤 (Ziyan 子言)||Jisun Zhaobo 季孫昭伯 (Ji Fanghou 季魴侯)||Ji Ziran 季子然||Gongfu Wenbo 公父文伯 (Gongfu Chu 公父歜)|
|Gongchu Ji 公鉏極||Ji Kangzi 季康子 (Jisun Fei 季孫肥)||Nan Ruzi 南孺子 (Zhizi 之子)|
|NN||Gong Sizhan 公思展|
|Ji Zhaozi 季昭子 (Jisun Qiang 季孫強)||Gong Hemiao 公何藐|
Sources: Chen Houyao 陳厚耀, Chunqiu Shizu pu 春秋世族譜 (Lianghu Shuyuan edition 兩湖書院正學堂本). Wang Wenyuan 王文源, Chunqiu shizu jilüe 春秋世族輯略.