Here is the full list of all 19 Sephonie achievements.
Visit the postgame room.
Finish the epilogue.
Finish the game.
Enter the core of Sephonie Island.
Reach the depths of Layer 2.
Link with 20 non-key creatures
Reach the depths of Layer 3.
Find 20 non-key items.
Reach the depths of Layer 1.
Get the Return to Surface ability.
Link with Pomelia.
Link with 10 non-key creatures
Link with Thraskias.
Enter the first layer of the caves.
Link with a species on the surface.
Link with a Coral.
Link with all 29 non-key creatures.
Find 40 non-key items.
Find all the non-key items. Nice work! I hope you had fun. Why not drop by our Discord and say hi? - Melos