Dona is an original character created for Legendary Guardian and has no basis in cannon from Square Soft in Final Fantasy X.

Dona appears in only one chapter of Legendary Guardian. She is a young temple Acolyte who delivers a letter to Yuna. The Lady Yuna comments that she once knew another person with the name Dona – a Summoner. Those who have played FFX will recognize this as a reference to a side character that appears several times during the game.

I had no real description in mind for Dona, aside from the given information that she was an Acolyte – which identifies her as being between the age of 7 and 12.