Here is the full list of all 19 Sephonie achievements.
Link with a Coral.
Link with a species on the surface.
Enter the first layer of the caves.
Link with Thraskias.
Link with Pomelia.
Get the Return to Surface ability.
Reach the depths of Layer 1.
Reach the depths of Layer 3.
Reach the depths of Layer 2.
Enter the core of Sephonie Island.
Finish the game.
Finish the epilogue.
Visit the postgame room.
Link with 10 non-key creatures
Link with 20 non-key creatures
Link with all 29 non-key creatures.
Find 20 non-key items.
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