Unlocking all the characters can take a lot of time, as nearly all of them must get purchased with stars that you earn throughout the World Tour.
In World Tour, you get awarded stars depending on what difficulty you complete a challenge. You only get one star for completing a challenge on easy, whereas you get 2 stars for medium, 3 for hard difficulty and 4 stars for completing it on Expert. You may have to complete a few challenges on Expert in order to have enough to unlock all the characters, but you might get enough if you do quite a few on hard. I believe there are 58 challenges in total.
The last character you'll unlock in the World Tour is called AGES and he costs 165 stars, so if you have that many stars you'll also have enough to unlock all the rest of the characters in the World Tour if you haven't done so already.
There's also Dr Eggman, who can be unlocked by completing the Grand Prix.
Here's a list of all the characters you need to have unlocked:
Sonic, Tails, Knuckles, AiAi, Beat
Amy, B.D. Joe, MeeMee, Ulala, Ralph
Amigo, Danica Patrick, Shadow, Vyse, Dr. Eggman
NiGHTS, Gum, Joe Musashi, Pudding, Gilius
Avatar, Reala, AGES