A summary of the class, rank and title system in the Twelve Kingdoms can be found in this note to A Thousand Leagues of Wind. The Daiboku is a lower-ranked baron (下大夫).

Koshou makes his first appearance (by name) in chapter 45 of A Thousand Leagues of Wind. Youko encounters him earlier, but doesn't know his identity.

路寝　[ろしん]　Roshin (path + sleep)
正寝　[せいしん]　Seishin (righteous + sleep)
大僕　[だいぼく]　Daiboku
虎嘯　[こしょう]　Koshou (tiger roar)
劉李斎　[りゅうりさい]　Ryuu Risai (axe + plum + purification)
騎獣　[きじゅう]　kijuu (riding horse + beast)

Labels: 12 kingdoms, shore in twilight