Boshi is a Blue Yoshi who lives on Yo'ster Isle with the other Yoshis. Boshi is somewhat different in appearance to the other Yoshis of the game, wearing sunglasses, a spiked red collar, and shoes, which allow his three toes to poke out, instead of the usual Yoshi shoes. He also has a red crest protruding from the back of his head. He is a bully and the fastest runner in the Mushroom Derby. Boshi later gets beaten by yoshi and they become rivals. In the Japanese version, his name is Washi (ワッシー Wasshī?), similar to Wario. This is both a play on names and words, like Waluigi, combining bad (悪い warui?) & Yoshi (ヨッシー yosshī?).
During the ending of the game, when Croco loses to Yoshi, Boshi meets Croco and gives him solace as they walk away from the celebrations. Boshi has only appeared in Super Mario RPG: Legend of the Seven Stars.