It has been speculated that the ribbon on Rumia's head acts as a seal on her true powers.
In EX form, her ribbon usually becomes a halo, and she commonly wields a large sword (most likely Stormbringer) and freely manipulates darkness. She is also sometimes depicted as an adult and/or with wings in this form. It is also not uncommon to see her ribbon floating above her head in a similar fashion to a halo. She becomes more mature and more serious in this form.