Wing Upgrade System
The best wings in the game currently drop from the Favnil Nest Dungeon. To get to this dungeon, Ardreca -> Valley Rift -> Dravice Village (Town) -> Favnil's Nest (it shows as Ancient Presence [InDun] in the Map).
These are the PVP Offensive and the PVP Defensive Wings. The last boss of the dungeon drops the Best Wings Randombox which may give an Offensive Wings Box and a Defensive Wings Box. From the Offensive Wings Box, you may choose a Physical version and a Magical version.
There are 4 Wing Ranks. Base -> Eo -> Xera -> Ys. What you get from the Dungeon is the base version already, meaning you have to upgrade only three times. To upgrade, you will be needing Liberty Essence. 100 of these are required per upgrade regardless of the Wing Rank. Talk to any Blacksmith NPC to begin the upgrading process.
Base -> Eo and Eo -> Xera have a 100% success chance. Xera -> Ys only has a 5% success chance. However, failure will not have an effect on the Wings. You will just lose the essence and the dil used for the upgrade.
To change the Wing Appearance, visit Viola in Ardeca. You may freely change from one style to the next. This will cost 70 Mystical Feathers and 1M dil. Mystical Feathers can be bought via D-shop. This is the only way of getting the feathers for now.
Massive thanks to Nidu and SmalleR. None of the guides would have been done as quick without them.