As we enter the third Skylanders game things get a little more complicated, but it's really not that bad.
Below you will find all the characters that are returning from either Skylanders Spyro's Adventure, Skylanders Giants, or both.
If the character has only been in one of the two previous games, it will have a Series 2 figure (i.e. Chill). If the character has been in both of the previous games, it will have a Series 3 character (i.e. Stealth Elf).
As they are returning characters, they should all be compatible with previous games.
Click the pictures to get more information and a larger image.