Here is the full list of all 21 Kind Heart Defenders achievements.
Complete the tutorial
Clear a stage without taking damage
Win with the green ship
Destroy 1000 enemies
Destroy 8000 enemies
Destroy 15000 enemies
Win with less than 5% of the ship's HP
Win with less than 5% of the heart's HP
Make a maximum upgrade
Win with the pink ship
Win with the blue ship
Win with the yellow ship
Build 100 towers
Build 250 towers
Build 500 towers
Collect 5000 coins
Collect 15000 coins
Collect 30000 coins
Use 10 power-ups
Use 50 power-ups
Use 100 power-ups