Of all the characters in the game, she is the only "new" one, meaning she is the only one being introduced in the game, much like how Toadette was introduced in Mario Kart: Double Dash‼ for the GameCube.
Baby Daisy has starred in three Mario spin-off games as an unlockable character. The first game she starred in was Mario Kart Wii. She was one of the four baby characters in the game. To unlock her in Mario Kart Wii, a rank of star or higher on all new 50cc courses is needed. She appears in Mario Kart 8 as well returning as a lightweight character, though now unlocked from the start.
This cute toddler made her first appearance in Mario Kart Wii. If she looks familiar, that's because she's a baby version of Princess Daisy! Look at the determination in those baby eyes --- even at such a young age, she was full of drive! Orange was her favorite color at that age too. Even her pacifier is orange!