While in middle school, Koga began studying painting with Matsuda Teisho, a self-taught local artist inspired by Western-style art.

Local tradition in Kurume records that Koga purposely violated the rule and got himself expelled in order to show his family how serious he was about pursuing his love of art.

[2] That same year, at age 17, Koga left Kurume against his family's wishes and traveled to Tokyo with the intent of studying art in a more formal setting.

Later that year, Koga and Matsuda Teisho formed the Raimoku Western-style Painting Society with a number of local artists in Kurume.

After a brief stay in Nagasaki, and a thwarted affair with a distant relative of his father, he entered the Buddhist priesthood and was given the name Harue ("spring bough," usually a feminine name).

Shortly after, his father died and he began auditing classes in theology at what is now Taishō University while continuing to paint in his spare time.

Although in critical condition, he recovered but had missed so much time at the university, he decided to quit school and concentrate on painting, after which his family withdrew financial support.

[7] Koga's love of Asakusa Opera was generally disapproved of by his colleagues in the Japanese Watercolor Society, as it was as considered a low-class art.

Around this time, Koga became deeply interested in emerging schools of thought and was introduced to painters Seiji Tōgō and Kongō Abe, who had just returned from studying art in Europe.

Recent scholarship has uncovered how Koga appropriated a wide variety of images and motifs from contemporary mass-media sources such as photo magazines, newspapers, film stills, and postcards.

However, Koga's technique was unique in that he did not simply cut and paste from printed media onto his canvas, but instead modified, resized, and rearranged objects by painting them photorealistically onto his large canvases.

Following his death, Koga's acquaintance Nakano Kaichi, who was a doctor, chronicled his symptoms and conducted a psychiatric evaluation.