Mario

Mario is the main character from Nintendo's flagship Super Mario series, his namesake being Mario Segale, the then-landlord of Nintendo of America's storage and company housing. Like Yoshi, Mario has made cameo appearances in several Metal Gear games, specifically those that were developed for Nintendo systems.

Metal Gear Solid: The Twin Snakes
Mario appears in Metal Gear Solid: The Twin Snakes as a cameo along with Yoshi in Dr. Emmerich's laboratory as a pair of dolls. If the player shoots Mario, a "1-Up" sound can be heard and Solid Snake will gain a little bit of health. If the player shoots Yoshi, he'll say his name.

Super Smash Bros. Brawl
Mario and Solid Snake both appear as characters in Super Smash Bros. Brawl. During a Codec conversation with Colonel Campbell, Snake expresses familiarity with him due largely to his massive fame in video games since his debut in Donkey Kong in 1981, and indicates he was looking forward to an opportunity to fight him with everything he has. Said codec conversation was also used for Snake's debut trailer for the game.