Here is the full list of all 24 Ukko & Guará: Stellarbound achievements.
Let Guará faint from the cold
Let Guará faint from the cold 10 times
Let Ukko faint from the heat
Let Ukko faint from the heat 10 times
Pass the star 10 times
Pass the star 50 times
Pass the star 100 times
Throw the star into the portal 10 times
Throw the star into the portal 50 times
Throw the star into the portal 100 times
Press the button 10 times
Press the button 30 times
Press the button 50 times
Plant 10 seeds
Plant 30 seeds
Plant 75 seeds
Ricochet the star 10 times
Ricochet the star 30 times
Ricochet the star 50 times
Build 3 bridges
Build 10 bridges
Build 25 bridges
Start the game
Finish the game