|Real Name||Ko Zen|
|“||Mou Ten-sama...Being able to meet you brought this old man so much joy in this life. Thank You...||”|
—Ko Zen's last words.
Ko Zen was the adjutant of the Gaku Ka Unit.
Ko Zen was an old man with a bushy mustache and a short bushy goatee.
He had a courteous personality. Also, he is an overtly-emotional one, especially on his young master's wellbeing, from his master's choice in life, to small victories. He is extremly loyal to Mou Ten and was overseeing his grouth, since early childhood.
Ko Zen was Mou Ten's guardian, raising him since early childhood and later became his adjutant. Mou Ten really cares deeply about him, even calling him "Gramps". Ko Zen was also very loyal to him, not hesistating to try to drive away Hou Ken before Mou Ten notices what was going on and arrive there.
Under orders of Mou Bu, Ko Zen has raised Mou Ten since the day he was born, developing a loving relationship with the young heir of the Mou Family to the point that he was fondly called "gramps" by him.
Ko Zen first appeared commenting on the competition for achievements between the Hi Shin and Gyoku Hou units. When Mou Ten declined to join the competition because he deemed that arena too tiny, Ko Zen agreed and offered to find a grander battlefield for his protegée. Mou Ten declined once again saying he didn’t felt motivated to do anything with only 300 men based on his achievements in the Anhou Campaign. Ko Zen then remembered that Haku Rou orders were for Mou Ten to gain some additional years worth of experience, and that his orders could not be disobeyed.
When he overheard Mou Ten sayin his aim was to be a Great General of the Heavens, the old "Gramps" was overjoyed to the point of tears. However, he was quickly disappointed when Mou Ten complemented his speech, saying he was just joking and that he rather led a simple officer's life.
As the adjutant of famous Gaku Ka Unit, Ko Zen was a talented and experienced commander, playing important supporting roles during battles.
On the battlefield, Ko Zen was capable of wielding both glaive and sword. Before his death, he showcased enough skill and willpower to injure Hou Ken's leg with a surprise attack.
Ko Zen was able to command and rally men effectively on the battlefield. Mou Ten and men in Gaku Ka Unit were devastated to hear about his death during the Battle of Shukai Plains.
- Ko Zen's name was not revealed in the manga until Chapter 520. However, before that it was shown in both guidebooks.