Aceso was originally a Mage In-Training in Radiant Garden and accidentally becoming a Keyblade Wielder. After her Homeworld was invaded by the Heartless she ended up in Daybreak Town before the Keyblade War. Right now she is travelling through worlds to help those in need.
From an early age Aceso had the ability to sense the Light and Darkness of people inside their hearts and understanding who whose Good or Bad. She was a Mage In-Training and sometimes one of her spells would end up into an explosion but she was skillfull on casting Healing spells. She knows the legend about the Keyblade war and the existance of other worlds which she learned from her family especially from her brother Aratus.
Kingdom Hearts Birth by Sleep
One day she was on her way to return some books in the library when she felt the presence of a strong light in a person's heart which didn't have darkness at all and few others who among them had strong darkness. While running to invistigate the mystery she didn't watch where she was going and fell into someone and her hand grabbed something. When she open her eyes the first thing she saw was a strange shaped sword looking like a key, she recognised as keyblade and then she looked to the person she fell which was a mouse.
The mouse who introduced himself as Mickey asked her if she saw a boy with blonde hair, Aceso said she didn't see him but maybe he could look near around the castle. Mickey thank her for her help and running in the pointed direction before telling her to be careful for a boy in a mask or any blue creatures. The girl before continuing her way decided to return the books and then invastigate further.