Kore

The "Escape"-Achievement will automatically unlock once you start the game - not missable

The "Passage"-Achievement will automatically unlock once you pick up the coin in the middle of the story - not missable

The "Elysium"-Achievement will automatically unlock once you have fully completed the game - not missable

The "Charon's Apprentice"-Achievement will unlock once you entered the dev-mode and became a boat... yes... a boat. To do this simply press Alt+"d" while the game is unpaused in-game, which will result in you entering the dev-mode. Afterwards simply press Alt+"b" to become a boat.

Congratulation! You are now a boat and completed all of the achievements!