Yoshi (ヨッシー, in japan) is one of the heroes of Mushroom World and an ally of Mario. He has also saved other kingdoms from the villain Bowser as well. Yoshi has a special relationship with Birdo, since they are often partners in sports and party games. The ring Birdo wears is suggestive, but likely unconnected. In Mario Kart: Double Dash!! the two are seen on the Turbo Yoshi, also they share a similar item. On the official site for Mario Kart: Double Dash!! the description for Catherine states "Catherine appears to be Yoshi's girlfriend...Or does that mean boyfriend?!!.
In Mario Tennis for the Nintendo 64 they are default partners and the two are seen to be very affectionate. Also they share a Tennis Court. This court favors tall and fast players. In Mario Superstar Baseball they have a compatible relationship, she was found on Yoshi's team as a unlockable Sub-Captain, and when they are challenged, they are versed in the Yoshi Park.
In Mario Super Sluggers, Birdo is seen during the game's opening with Yoshi on the Yoshi Park. They also shared good chemistry. In the Challenge Mode, once Yoshi enters to his Park, he must to save Birdo from Bowser Jr. Yoshi then rescues Birdo, and she accompanies him. Afterwards, Birdo is able to join the Yoshi Eggs to assist in saving the rest of the Baseball Kingdom. At the end of Challenge Mode, they both are seen running to toss Mario in the air as congratulations for defeating Bowser. In the Mario Party installments they're teammates, their team orb being the Egg Orb and their team name is: Egg Explosion. They also were seen nuzzling each other on the GameCube Boxart of Mario Party 7 as well as they appear at Mario Stadium in Mario Hoops 3-on-3.