Sage is Sensei's puffle who liked to meditate and train with him at Serene Springs. His skills as a puffle were reported to surpass Sensei's skills sometimes. When a curse was set on the grey puffles, Sage turned to stone and remained like that for years. Sensei believed he didn't want to force Sage free and that waiting would be a better option.
After many years of being stuck as a statue, the curse was lifted, and Sage was freed. He returned to Sensei in 2019, around the time the sakura trees were blossoming.
It is currently unknown how this curse turned Sage and the other grey puffles to stone for many years. However Sensei said that more news about this will be revealed.