Sworn younger brother to Liu Bei and sworn older brother to Zhang Fei. He is as fierce as Zhang Fei but is more restrained and calm in his mannerisms. 

Romance of the Three Kingdoms, written by Luo Guanzhong in the 14th century, is a Chinese historical novel based upon events in the turbulent years near the end of the Han Dynasty and the Three Kingdoms era, starting in 168 AD and ending with the reunification of the land in 280 AD.
Yokoyama Mitsuteru's adaption begins from the decline of the Han Dynasty until the fall of Shu-Han.
 
(Source: Wikipedia)

Around 1800 years ago in China, the ruling power of the Han dynasty became weakened and the flames of war spread throughout the land. 
Liu Bei was a young, poor straw-mat seller from the countryside of Hebai Hsing. After pledging eternal brotherhood with the two heroes, Guan Yu and Zhang Fei, Liu began building his idea of a righteous kingdom that would aid citizens who suffered from persecution. 
(Source: AniDB)

[Wiki (jp)](https://ja.wikipedia.org/wiki/辻親八)

A TV special animation of the Chinese historical romance that has been passed down through the ages to this day. China was the era of Eastern Han. Under the imperial court that lost power, the security of the world was disturbed, and a large group of bandits, the "Yellow Turban Party," was doing all the violence all over the country. Liu Bei Gentoku, a descendant of the Chinese king who was worried about the world, stands up with his brother-in-law, Guan Yu and Zhang Fei, who also want peace on the continent. Gentoku who pays victory with secret and strange measures so as to gradually cut down a group of mighty enemies, the Yellow Turban Party. Hero of Time ... 
 
(Translated source: allcinema>

Yu Guan