Here is the full list of all 20 Solo achievements.
Lay on the beach.
Photograph a red seagull for the Sapling and give it to him.
Help the shiters on the first archipelago.
Water the flowers on the first archipelago.
Complete the first archipelago.
Help the first shiters on the second archipelago to be together.
Help the last shiters on the second archipelago to be together.
Photograph a fish for the sapling and give it to him.
Water the flowers on the second archipelago.
Complete the second archipelago.
Help the first shiters on the third archipelago to be together.
Help the last shiters on the third archipelago to be together.
Water the flowers on the third archipelago.
Complete the third archipelago.
Complete the game talking on the bench.
Complete the game by boat.
Gather all the songs.
Feed all the animals in the game.
Interact with all the ghost partners.
Pet a hiding mole.