After completing the level Sky Skooner Docks you will be able to begin upgrading your Skylander with Persephone the fairy who is to the left of the hot air balloon.
To get this a single Skylander requires to have every skill purchased for them. Most have 8 skills that need to be purchased and will set you back a bit of money but even if you switch a lot you should have this prior to completing the game.
The trickiest part of this achievement is that every Skylander requires a soul gem to be found in levels that unlocks a final skill for them. To see if you have the soul gem for a character go to 'Collections' in the pause menu and then 'Skylanders' if the circle around your character is decorated with a 10 point star it means you have found it.
As a bonus you can find Winged Sapphires (Blue Butterflies) around the island and each gives you a 2% discount on upgrades. There are 10 in total and you can only get a new one every major chapter break or so.
Winged Sapphire Locations:
1. Next to Persephone after Stormy Stronghold
2. At the gate that opens on the beach after getting the Eternal Water Source
3. Inside the cave opened by oiling a robot on the beach
4. Up a series of bouncy pads in the water to the east of the bridge
5. Under rock where Arbo came from
6. Inside fence next to the Train opens after getting the Eternal Undead Source
7. Inside a cave near the Train, blow it up with a bomb
8. Inside a hole near the Train, blow it up with a bomb and then jump in
9. Oil the Train
10. Inside the prison by the docks after getting the Eternal Magic Source