Kotsuki is one of Hotohori's younger half-brothers and was his closest friend before Moutaikou came to power. He is a lord of Sairou, residing in Shikin Castle. He and his mother were sent away to this small, remote castle after the death of the previous emperor, Hotohori's father Kougentei. He travels to Kounan to visit and while waiting for Hotohori to choose horses for horseback riding, he is fed a poisoned bun by Moutaikou, which kills him. Hotohori returns, horrified by what he sees, exclaiming to his mother that Kotsuki was no threat to her or himself.
He appears in Hotohori's background novel Suzaku Hi Den.