To unlock you must simply complete the story mode beating all eight opponents using Maya on any difficulty earning any objectives.
If you want progress towards :
She has three objectives, and all can be completed in one run.
1) Beat story mode. Simply win all eight fights.
2) Get 100% Shadow Efficiency in one match. This can be annoying, as you need to attempt a minimum of four shadow attacks and land all four. The problem is earning meter. A nice trick is to land two, then cancel an ultra into two more once you have the required shadow again.
3) Ultra combo Kan-Ra. Simply beat Kan-Ra with an Ultra in the final fight. Her ultra is
+
when Kan-Ra's health is reading 'danger'.