Now, technically, the Queen of the Cherrystone kingdom doesn’t need to be unlocked. She’s the only commander available right from the start of Arcade Mode (which is itself unlocked by completing the first chapter of the campaign).
As is fitting for the first commander we get to control, she’s a solid
and reliable pick. Her Groove is Healing Aura, which heals all her units in a wide AoE around her. It restores 50% health to each unit in range and herself, and it charges quickly to boot.
This is very useful, as it gives her the ability to press an attack (and remain in the front line) like few other commanders can.