The most common instrument played alongside the koto instrument is the shakuhachi (bamboo flute). The shakuhachi made its presence in Japan from China during the Edo period as a form of Zen Buddhist meditation called suizen (blowing meditation). As described by Wikipedia, the Fuke sect of Zen Buddhist monks is known as komusō, or priests of “nothingness” or “emptiness”. The shakuhachi is most commonly made of bamboo, its original form. The term itself is an ancient unit of measurement. The instrument is extremely versatile and seemingly limitless in range. But the most distinctive part of the shakuhachi sound is its depth and richness of tone.
Here is a basic introduction of the bamboo flute, or shakuhachi, and koto music played in harmony together. The following are some I have in my collection: