She's an Ionian-born, Demacian. Only problem with that is that she's nowhere to be seen. She appears to be absent in the Ionian vs Noxus war and no where to be seen in Demacia, where she would either be imprisioned, exiled, or killed. Last time she was seen in lore, she was helping Ryze in Ionia.
Things to consider:
1.) Lux has hinted to knowing what Sona's instrument truely is, likely from her reading all those books on magic.
2.) She might has relations to Quinn, since her adoptive mother Lestara was saved by Quinn's brother.
3.) If we go by what Morgana says "Your soul sings for a fallen world, Sona." Sona may not be entirely innocent.