There are seven workers available in the game.
You start with the three main characters - Albert, Diana, and Jules. The next main character, David, you will find as you progress through the main storyline.
Tip: he is trapped in the sticky web. Ew!
Another two you can buy during the special offers in the game.
By completing the Timenarium archipelago with three stars you will be awarded an additional worker named Carrie.