Puu is Yusuke's Spirit Beast. He hatched from an egg Koenma gave Yusuke. Because Yusuke's heart was pure, so was the beast's.
Puu shares Yusuke's emotions and life. If Yusuke is sad so is Puu. If Yusuke dies, so will Puu.
He is taken care of by Keiko. He can fly and his only words are "PUU!" Puu can also make a protective barrier.
Yu Yu Hakusho and all associated characters are copyright of Togashi Yoshihiro, Studio Pierrot, Fuji TV, Shounen Jump Weekly, and Shueisha. It's licensed by FUNimation Productions, Ltd. This is simply a fan site to it.