Birdo is a mini-boss enemy that originated in Super Mario Bros. 2, frequently encountered at the end of a regular level or as a mini-boss before a world boss. Since then, Birdo has shown up at multiple Mario sporting events, adventures and party games.
In Mushroom Kingdom Fusion, Birdo's role reverts to that of her original role as a mini-boss. Her sole attack to spit eggs at her enemies. The player fights Birdo by grabbing the regular eggs she spits and tossing them back at her. Sometimes, the eggs will be fiery to prevent the player from grabbing them.
There was some initial confusion over Birdo's gender, as the Super Mario Bros. 2 instruction booklet referred to her as a guy who thought he was a girl. Later games simply refer to Birdo as female or as "gender indeterminate".