Technically, all achievements can be gotten in your first playthrough but this is unlikely. All achievements are permanently missable as the game doesn't offer a "post-game-cleanup"-option and some achievements also become unachievable throughout the end. The other achievements are listed in the way you can obtain them, to keep spoilers to a minimum for you.

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The following are story-related achievements which cannot be missed. I won't write about them since this is not a walkthrough of the entire game and the game is pretty straightforward.

The following achievements can be earned simply by completing the game:
- Mean Cuisine
- Digestive Troubles
- Clean and Tidy
- Time to Split
- DJ Ada
- Diamond in the Rough
- Sneaky Leaky
- Once Upon a Slime
- Round 'em up!
- Speak & Spell
- Kraken Me Up
- A-Maze-ing
- Escape the Nightmare

Rock Around the Clock Tonight
Right after you gain control of Ada, go to the top of the mansion and interact with the clock hanging on a rope. Then explore the mansion and gather 4 rocks, throwing them at the clock one after another. Once you've thrown the fourth and the clock falls down, you get this achievement.

I Need to Return Some Tapes
This is the most annoying and time consuming achievements. Every time you encounter the demon, K.O. him and leave the room before he teleports on his own, he will leave a tape. You then have to take this tape to the tape player in the bedroom in the top of the manor. There are a total of 6 tapes. This can be done as soon as you gain control of Ada.

The tape will only spawn if you get to an exit quick enough after you K.O. him. So lure him near to a screen transition, K.O. him, transition the screen, come back and pick up the tape.
You will know when the demon is in your room by hearing his heavy breathing. I could not entirely figure out what causes him to spawn but I think he spawns somewhere in the mansion after you got "scanned" by that weird orb-thing left of the garden, in the tool shed. Scan yourself like 5 times so it makes a noise, then explore the manor until you find the demon. Rinse and repeat.

Fashionista
There are a total of 6 costumes in the game.

- Robot: You find this in the changing room after getting the golden key from knocking the clock down from one of the previous achievements. There you can simply equip the robot.
- Dragon: Equip the Robot and go to the electrical ladder under the bedroom. Use your PSI-ability and the Robot will cause sparks to appear, enabling the ladder. Go up the ladder, into the attic, and grab the Dragon costume.
- Diver: Equip the Dragon costume, go into the garden and go to the top right. There's a secret passage obscured by the tree's crown. Once again use your PSI-ability, this time with fire, to burn the vines and get the Diver costume.
- Builder: Equip the Diver costume and go to the bathroom. Interact with the waterhole in the top right and play the little minigame. After about 50 seconds you get to the bottom and get the Builder suit.
- Teddy: Still have the diver equipped, go into the toy room on the left side of the mansion and use your water-PSI-ability on the giant teddy bear.
- Green Dress: You get the Green Dress automatically through storyprogression in the second dungeon.

Wanna Play a Game?
Follow the 'Fashionista' achievement until the Builder costume. Then equip it and head to the attic where you got the Dragon costume. There is a wall with a crack, use your PSI-ability there. Enter the room and grab the game cartridge. The achievement should unlock now and you don't actually have to play the game.

Vending Machine Wizard
This is an achievement you have to pay attention to for the remainder of the game. In each demon realm you need to defeat monsters until they drop a coin. Then go to the vending machine found in each realm and use it. After you used the machine in all demon realms, you get the achievement. Note that the coin IS NOT guaranteed to spawn so you may have to reload a save if you didn't get one by the end of the demon realm. Also, all demon realms have finite enemies, except the 4th which has respawning enemies.

Gotta Squish 'Em All
In the first dungeon you can encounter grubs / maggots. Simply walk over ten of them for this achievement.

Not On The Menu
I don't know if the devs intended this but this is actually NOT a "no-hit"-achievement. Go through the demon realm and before freeing the ghost, save at the mirror - this heals you to full health. Then you'll also get the achievement when you free the ghost, even if you took damage throughout the dungeon.

Who's the Goodest Boy of All?
After the first dungeon you get Gnarlie, a very good boy. Stand near him, press A to pet him. Repeat 50 times for the achievement, repeat 1000 times for eternal peace.

Cutting a Rug Tonight!
In the 2nd dungeon, in the room where you get the scissor, simply kill all the other monsters too!

Stop Poking Me!
Same procedure as the first full-heart achievement. Simply save at the mirror before freeing the ghost to regain your hearts.

You Spin Me Right Round Baby
Same procedure as the other two full-heart achievements. Simply save at the mirror before freeing the ghost to regain your hearts.

You Spin Me Right Round Baby
Same proce- wait actually not! This time the achievement doesn't require you to have full hearts at the end but rather requires you to not ram yourself into any spikes. Easy enough but I recommend saving in the beginning and reloading if you end up ramming one anyway.


And that's the last missable achievement. All other achievements are story-related and cannot be missed.

Hope you enjoyed and I hope anybody can help me & us with the Fashionista-achievement.