She's White Mage from Mario Hoops 3 On 3/Slam Basketball and Mario Sports Mix. She's technically a Final Fantasy Character, but because those two games where developed by Square Enix they had some Final Fantasy Guest Characters included (White Mage, Ninja, a Moogle and a Cactuar. Sports Mix also included Black Mage and a Slime from Dragon Quest as well).

Mario Hoops 3-on-3, which was developed by Square Enix (like Mario Sports Mix later on, where the White Mage also appears). On a side note, I love the artstyle they used for that game and it's a shame that it hasn't shown up in any other Mario game. Here's the White Mage's character portrait, for example.