Yoshi

" Hours, that's long time."- Super Mario World Cartoon- Mama Luigi

 Yoshi is a happy little dinosaur who is Mario's friend.

Super Mario World
Yoshi's land was trashed by Bowser, so he helps Mario on his quest to save Princess Toadstool. He can not go in ghost houses, fortresses, and castles.

Super Mario All-Stars
Yoshi appears as a transformed king in World 7.

Super Mario World 2: Yoshi's Island
Yoshi is the main protagonist of this game. He is found in the first, extra, and secret levels of each world, and W6-8.

Super Mario RPG: Legwnd of the Seven Stars
Mario rides Yoshi in a 3-D environment.

Yoshi
Yoshi appears in the corner making Q*bert quotes.

Yoshi's Cookie
Yoshi appears in Vs. Mode.

Yoshi's Safari
Yoshi's head can always be seen while Mario shoots.

Super Mario Kart
Yoshi is a playable character driving his Pipe Frame.

Mario Is Missing!
Luigi can ride on Yoshi to help him.

Mario's Time Machine
Yoshi appears in the NES version of the game.

Mario's Tennis
Yoshi appears as a tennis player.

Trivia

 * Yoshi is apparantly a lot older than Mario.