Ok, Im pretty sure I 100% the game.
THIS COMMENT INCLUDES SPOILERS TO (i think) ALL THE SECRETS IN THE GAME, so if you want to find them yourself - GO DO IT, it is fun.
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CATS
To unlock shop scenes - you have to find 4 hidden cats on the map, they make noise when you go near them, so youll find them in no time.
MAN BEHIND THE TREE
To find the (really cool) scene with the Man, you need to go back an forth beetween the room with the bookshelf, and the room to the right (the one with the endless void, and a door to the bedroom). The chance is really low, or I was just unlucky, but after like 2 minues you should find it.
When you enter the secret area youll find a tree, but then you have to go up, to find another tree, and then up again and to the right. From that point I dont think you can get lost, so just follow the path.
When you get back to the bedroom, you will have an egg in your inventory, which you can use, and crack by just pressing Z.
MYSTERIOUS ITEM
When I saw the Mysterious Item in the shop I immediately focused my attention on the weird wording of the description:
-" It's a mysterious"
After looking and not finding anything for 15 minutes I decided to try adding myself that item through the files - AND IT WORKED!
The weird description was a hint, the item in the code is literally called "mysterious". You can find your save file in the:
"[...]\AppData\Roaming\Godot\app_userdata\Love Maniac\save\slot_0.json"
First I added the "mysterious" into my key items, It worked, the item was there, but I couldnt do anything with it - so I tried to add it to my normal items, and It worked. You can use the "mysterious" during battle, and it will give you unique dialogue (with Dess...I think?).
To add the item into the files you just have to add "mysterious", (with the quotation marks) into your "inventory_items": [. Remember - all of your items have to have a comma at the end, EXCEPT the last one on the list, so it should look something like this:
"inventory_items": [
"legendary_hero",
"star_cookie",
"gelly_fish",
"mysterious",
"strange_pill"
],
Using the "mysterious" in the battle unfortunetly doesnt unlock anything besides the dialogue, unless I missed it - but Im 99% sure that its just a cool puzzle witha a teaser for later updates, because after using the item the game crashes to the menu, and your save file doesnt change.