One of Koga's most popular pieces, Hooyoroi Hai, uses the motif of the unusual helmets of warlords from the Sengoku period to express the way of life and spirit of people.

``Today's era is like an era of subjugation,'' says Koga.
With the development of social networking sites and the ability for everyone to have a say, we have entered an era in which people can aim higher than their origins or the area in which they were born and raised, as long as they have the will and courage to speak out. Within this worldview, Cheekyori Sakazuki is a work that uses the unusual helmets of Sengoku warlords as motifs to express people's way of life and spirit. It is also a shout out to the aspiring military commanders living in today's world.

This work was inspired by Takeda Shingen's ``Tiger of Kai'', a representative legendary Sengoku period warlord, and Uesugi Kenshin's nickname ``Echigo Dragon''.