Start by combining agar-agar powder and water in a medium pot over medium heat. Stir until the powder completely dissolves, about 3-4 minutes.
Pour in your chosen flavoring extract and keep stirring until the mixture starts to boil. Cook until it thickens slightly, about 5 minutes. You'll know it's done when the liquid coats the back of your spoon.
Pour the hot mixture into a rectangular pan. Let it cool on your counter until it becomes firm, about 1 hour. You can test if it's ready by gently touching the surface - your finger shouldn't leave a mark.
After it's firm, use a sharp knife to cut it into thin strips. First cut in straight lines one way, then cut across to make long, thin pieces.
In a large bowl, combine your drained fruit cocktail, all-purpose cream, and condensed milk. Mix these together gently until everything is well combined.
Add your cut-up agar-agar strips to the cream mixture. Using a spatula, fold everything together carefully so you don't break the strips. If the mixture feels too thick, add 2 tablespoons of the fruit cocktail syrup.
Cover your bowl with plastic wrap and put it in the refrigerator for at least 2 hours or overnight. Serve it cold, and if you like, top with a cherry before serving.