Divide the blueberries, the raspberries, and the mint leaves between two serving glasses. Using a muddler or the back of a spoon, gently press the ingredients to release their flavors. Avoid over-muddling the mint to prevent bitterness.
Add 1/2 to 1 tablespoon of agave syrup to each glass, adjusting the amount to suit your preference.
Squeeze the juice of half a lime into each glass.
Fill the glasses with ice cubes for a chilled drink.
Top each glass with 1 cup of soda water.
Stir well to blend all the flavors and evenly distribute the ingredients.
Finish with a sprig of mint and a lime wedge. Serve immediately.