Okaaaay, so, I just realised today that I have been a complete idiot and didn't actually write up a walkthrough for the most recent demo version of the game >.< It just totally slipped my mind and I can only apologise! But I'm typing it up now because I know with all the extra choices and stuff added in the demo update, it can make it kinda tricky to reach content that was previously much simpler to get to. I even confuse myself sometimes >.<"

If the version on this page is too hard to read (I know I really need to sort the page out T_T), I included a plain text document version of the same guide below as a downloadable file on the game page ^-^ it's the one called 'Darling Duality - Demo Walkthrough' Also, user ijedi1234 made this extremely handy flowchart guide, which is a heck of a lot easier to follow than mine! (thank you :3)

Since it's a walkthrough, it's inevitable that there will be some spoilers below, but I will try my best to be as vague as possible with that sort of thing + anything major I'll try to hide under a spoiler dropdown :3

KEY:

KIND = the HEART button (mainly impacts MC's personality/moral score and unlocks exclusive choices further down the line)

CRUEL = the SKULL button (mainly impacts MC's personality/moral score and unlocks exclusive choices further down the line)

Timed choices note - When a timed choice pops up, you don't have to select one of the available options, doing nothing and allowing the timer to run out is a choice in itself and will lead to different consequences.

SANITY - Some choices are not kind or cruel choices, these choices will often impact your love interest's sanity.

The general rule is that if you want to aim for the Sweet Side of a character's route, you pick mostly KIND choices and options that will keep your love interest's SANITY as stable as possible.

Whereas if you want to aim for the Sinister Side of a character's route, you pick mostly CRUEL choices and options that will lower your love interest's SANITY.

HOWEVER - There are exceptions to the above, and mixing choices and points can lead to various different outcomes. For example:

-You may find that you can escape a bad situation you've gotten into by having previously been a KIND enough person-

-If you pick too many CRUEL options or chip away at a love interest's SANITY too much, you may end up running into a BAD END-

-As the story progress and you build up points, some characters will notice if you try to LIE to them, so picking certain choices might BACKFIRE-

All right, here we go!

Step-By-Step Guide Part 01 - Common Route

The first choice you encounter in the demo is a TIMED choice when your phone rings. You can choose to answer or let it time out.
(In the future, this will matter, but in the current demo, even if you answer the call, you won't be able to proceed with the story branch yet!)

If you do answer the phone, there will be a few more choices.
(Again though, these choices don't currently lead anywhere. The only reason I left them in was because it was easier than deleting code I'd already finished, and I don't have enough space on my PC to keep making lots of backups >.<)

The next choice asks you what you want to do. Currently, the only option that will take you anywhere is [Play a visual novel]
(The other choices are there so you can see how other future routes will begin. In my working copy, a couple of them do go on to more, but in the current demo, they don't lead anywhere yet!)

Once you have selected the option to [Play a visual novel], you will then be able to Pick Your Poison
(Choose whether you want to play with Castor or Castoria)

- Castor/ia Route - 

The first choice you will encounter is a non-timed Cruel or Kind choice.
"Who's your favourite love interest in the game so far?"
Kind: +1 to MC's moral score
Cruel: -1 to MC's moral score

The next choice is another non-timed Cruel or Kind choice.
"Do you love me?"
Kind: +1 to MC's moral score
Cruel: -1 to Mc's moral score

The next choice is a non-timed Sanity choice.
"Should I take a peek at the bonus content?"
Look at the bonus = Sanity +1
Leave the bonus = Sanity -1

The next choice is a non-timed Sanity choice.
"I need to hear you say it..."
Promise = Sanity +1
Refuse = Sanity -1

The next choice asks if you want to carry on playing Winter Wish or call your friend, Cana. In the current demo, continuing to play Winter Wish is the only way to proceed.
(Choosing the other option will have consequences in the future but is currently a dead end.)

The next few choices involve clicking on buttons. It doesn't matter which ones you pick, they will all advance the story with the same consequences.

The next choice is the first TIMED Cruel or Kind choice.
"Uhm, can I help you?"
Kind: +1 to MC's moral score
Cruel: +1 to MC's moral score
Time out: Sanity -1

The next choice is a non-timed Sanity choice.
Not with you = Sanity -1
I don't = No change
I do = Sanity +1

The next choice is the first TIMED Sanity choice.
Question him/her = Sanity +1
Push him/her away = Sanity -1
Time out: Different dialogue, but no score change

The next choice is another TIMED Cruel or Kind choice.
"You must think I'm terribly ugly to not even want me near you..."
Kind: +1 to MC's moral score
Cruel: -1 to MC's moral score
Time out: Sanity -1

The next choice is a non-timed Cruel or Kind choice.
"Usually, you'd be happier to see me."
Kind: +1 to MC's moral score
Cruel: -1 to MC's moral score

The next choice is another TIMED Sanity choice.
"Why don't you close your eyes, and you'll find out..."
Comply = Sanity +1
Refuse = Sanity -1
Time out: No score change, but different dialogue, prompted to choose again.

If you allowed the timer to run out, you will be given the same choice again. Choosing [Comply] or [Refuse] is the same as above.
If you allow the timer to run out again: No score change, more different dialogue, prompted to choose again.

If you allowed the timer to run out twice, you will be given the same choice again. Choosing [Comply] or [Refuse] is the same as above.
If you allow the timer to run out again: No score change, more different dialogue, prompted to choose again.

If you allowed the timer to run out three times, you will be given the same choice again. Choosing [Comply] or [Refuse] is the same as above.
If you allow the timer to run out this final time: Sanity -1

The next choice is a non-timed Cruel or Kind choice.
"It's like you don't even know me at all..."
Kind: +1 to MC's moral score
Cruel: -1 to MC's moral score

The next choice is a non-timed Sanity choice.
And who is that?: Sanity +1
But you're just a character: Sanity -1

The next choice is a non-timed Sanity choice.
Why me?: Sanity +1
Were you stalking me?: Sanity -1

!!!Below is where things can get kinda confusing!!!
What you do with the next choice will determine whether you miss the old bathroom scene on the sinister side & the old steamy scene too.
Where you end up from here will depend on a mixture of what your MC's current moral score is and what Castor/ia's current sanity is, combined with the choice you make next.

The next choices will also differ slightly depending on whether you previously called Castor/ia boring or not!

IF YOU DIDN'T CALL CASTOR/IA BORING/DULL:

The next choice is a TIMED Cruel or Kind choice.
"Are you sure you're okay? Because I'd be more than happy to take a look at you. Maybe I can help."
Kind: +1 to MC's moral score - LEADS TO SWEET SIDE - The day will end
(if MC's moral score & Castor/ia's Sanity are high enough - if not, will lead to BAD END 03 further along)
Cruel: -1 to MC's moral score - LEADS TO SWEET SIDE - The day will end
(if MC's moral score & Castor/ia's Sanity are high enough - if not, will lead to BAD END 03 further along)
Time out: Sanity -1 - The day continues
(But whether or not you end up on the sweet or sinister side will still depend on further choices & current point scores)

If you picked the KIND choice above, you will then come across a non-timed Sanity choice.
"Will you come with me?"
No: Sanity -1
Sure: Sanity +1
(Whichever choice you pick here, the day will end, and you will be walking the Sweet Side path)

If you picked the CRUEL choice above,  you will then come across the same non-timed Sanity choice as above.
"Will you come with me?"
No: Sanity -1
Sure: Sanity +1
(Whichever choice you pick here, the day will end, and you will be walking the Sweet Side path)

IF YOU DID CALL CASTOR/IA BORING/DULL:

The next choice is a TIMED Cruel or Kind choice.
"Did you really mean everything you said earlier?"
Kind: +1 to MC's moral score - The day continues
(But whether or not you end up on the sweet or sinister side will still depend on further choices & current point scores)
Cruel: -1 to MC's moral score - The day continues
(But whether or not you end up on the sweet or sinister side will still depend on further choices & current point scores)
Time out: Sanity -1 - The day continues
(But whether or not you end up on the sweet or sinister side will still depend on further choices & current point scores)

Whether you called Castor/ia boring/dull or not, if you didn't pick any of the options that caused the day to end, you will next encounter another TIMED Sanity choice.

Stay away!: INITIATES POINT CHECK
FAILS if Castor/ia's total Sanity points are LESS THAN 0 (-1, -2, -3, etc.) - Leads to Sinister Side
SUCCEEDS if Castor/ia's total Sanity points are 0 or higher (0, 1, 2, 3, etc.) - Leads to Sweet Side

Come back to me!:  INITIATES POINT CHECK
FAILS if Castor/ia's total Sanity points are LESS THAN 0 (-1, -2, -3, etc.) - Leads to Sinister Side - [triggers a condition that leads to BAD END 02 later on]
SUCCEEDS if Castor/ia's total Sanity points are 0 or higher (0, 1, 2, 3, etc.) - Leads to Sweet Side

Time out: Sanity -1 different dialogue, prompted to choose again.

If you allowed the timer to run out, you will be given the same choice again. Choosing [Stay Away!] or [Come back to me!] is the same as above.
If you allow the timer to run out again: Sanity -1 more different dialogue, prompted to choose again.

If you allowed the timer to run out again, you will be given the same choice again. Choosing [Stay Away!] or [Come back to me!] is the same as above.
If you allow the timer to run out again: Sanity -1 more different dialogue, prompted to choose again.

If you allow the timer to run out for the last time: 
If Castor/ia's total Sanity points are LESS THAN 1 (0, -1, -2, etc.) - Leads to [BAD END 01 - You're still not smiling]
If Castor/ia's total Sanity points are 1 OR MORE (1, 2, 3, etc.) - You will be prompted to choose either [Stay Away!] or [Come back to me!] without a timer this time.

SUCCEEDED IN CALMING CASTOR/IA DOWN

(If you failed the above point check and Castor/ia is still acting crazed, you can scroll down until you reach the next header text 'Failed to calm Castor/ia down')

If you successfully got Castor/ia to calm down by passing the Sanity point check with the choices above, the next choice you will encounter is a non-timed choice.
Encourage him/her: Encourages Castor/ia's steamy behaviour, leads to further choices [Use this option to BEGIN the steamy/suggestive scene!]
Get him/her to cool off: Calms Castor/ia down a little and leads to a further choice [Use this option to AVOID the steamy/suggestive scene!]

If you picked [Get him/her to cool off] for the above choice, you will get another choice.
No: Avoids kissing Castor/ia - no point-related consequences - story continues - Leads to a further choice where Castor/ia will ask if you will go to bed with them [picking No = Sanity -1 picking Yes = Sanity +1]
Yes: Leads to kissing Castor/ia but avoids the steamier parts of the scene

If you picked [Encourage him/her] for the above choice, you will proceed to the steamy/suggestive scene and further choices.
Stop: Stops the steamy/suggestive scene from proceeding any further and continues the story
Keep going: Carries on with the steamy/suggestive scene

If you picked [Keep going] for the above choice, you will get presented with the same choices once again.
Stop: Stops the steamy/suggestive scene from proceeding any further - story continues - Leads to a further choice where Castor/ia will ask if you will go to bed with them [picking No = Sanity -1 picking Yes = Sanity +1]
Keep going: Carries on with the steamy/suggestive scene

If you picked to [Keep going] both times for the above choices, you will next get a TIMED choice.
Continue: You have 5 seconds to select this choice and continue the steamy/suggestive scene
Time out: Ends the steamy/suggestive scene

If you successfully hit [Continue], you will get another TIMED choice.
Explain: This time, you only have 3 seconds to select this choice and continue the steamy/suggestive scene
Time out: Ends the steamy/suggestive scene

If you successfully hit [Explain], you will get another TIMED choice.
Give in: You have 3 seconds to select this choice and continue the steamy/suggestive scene.
Time out: Ends the steamy/suggestive scene - story continues - Leads to a further choice where Castor/ia will ask if you will go to bed with them [picking No = Sanity -1 picking Yes = Sanity +1]

If you successfully hit [Give in], you will get another TIMED choice.
Stop: You have 3 seconds to hit stop.  Ends the steamy/suggestive scene - continues the story
Time out: Wait for the 3-second timer to elapse if you want to view the final part of the steamy/suggestive scene

If you made it through the scene without ending up going straight to bed, the next choice you come across will be a non-timed Kind or Cruel choice.
Kind: +1 to MC's moral score [LEADS TO SWEET SIDE]
Cruel: -1 to MC's moral score - story continues - Leads to a further choice where Castor/ia will ask if you will go to bed with them [picking No = Sanity -1 picking Yes = Sanity +1]

If you successfully made it through the scene, you will reach the Sweet Side path.

FAILED TO CALM CASTOR/IA DOWN

If you failed the point check when telling Castor/ia to [Stay away!] or [Come back to me!] If you picked [Stay Away!] you will get a choice that is missed if you picked [Come back to me!] instead.

It is a non-timed Cruel or Kind choice.
Kind: +1 to MC's moral score
Cruel: -1 to MC's moral score [triggers a condition that slightly alters later dialogue]

During the scene in the bathroom, if you picked [Come back to me!] earlier on and failed, you will get [BAD END 02 - Show me everything] here.

If you successfully made it through the scene, you will reach the Sinister Side path.

- Sweet Side - 

Depending on the previous choices made, you will either wake up in bed with Castor/ia, or on the sofa alone.

If waking alone on the sofa.
MC's moral score = less than 2: Leads to [BAD END 03 - Cruel Karma]
MC's moral score = 2 or higher: Story continues - Then end of demo reached on this path

If waking in bed with Castor/ia, you will get a non-timed choice.
"I can make you comfortable. If you'll let me..."
No: Story continues  - Then end of demo reached on this path
(in the future, this choice will likely have consequences, but in the demo, it currently does not!)
Please do: Story continues, leads to more choices

The next choice is a non-timed Sanity choice.
Promise: Story continues  - Then end of demo reached on this path
(this choice will eventually have consequences, but in the current demo, it does not!)
Refuse: Story continues  - Then end of demo reached on this path
(this choice will eventually have consequences, but in the current demo, it does not!)

- Sinister Side - 

The first choice you will come across is a non-timed choice.
I hate you: Leads to an HONESTY check on MC's moral score.
PASS if MC's moral score = less than -3 (-4, -5, -6, etc.) FAIL if MC's moral score = -3 or more (-3, -2, -1, 0, 1, 2, etc.)
If PASSED: Sanity -1
If FAILED: Story continues  - Then end of demo reached on this path

I still like you: Leads to an HONESTY check on MC's moral score.
PASS if MC's moral score = more than -4 (-3, -2, -1, 0, 1, 2, etc.) FAIL if MC's moral score = -4 or lower (-4, -5, -6, -7, etc.)
If PASSED: Story continues - Then end of demo reached on this path
If FAILED: No points consequences, story continues

If you don't end up on one of the branches that the demo ends on, you will next encounter a TIMED choice.
"Please, I'm begging you. Just this once, okay?"
Give in: You have 6 seconds to select this option and save yourself some pain - Story continues - Then end of demo reached on this path
Time out: If you allow the timer to run out, you will be presented with the same choice

If you let the timer for the above choice run out.
Give in: You have 3 seconds this time to save yourself some pain - Story continues  - Then end of demo reached on this path
Time out: If you allow the timer to run out  - Story continues - Then end of demo reached on this path
(allowing the time out currently has no consequences in the demo, but will likely have a greater impact in future updated versions of the game!)

And there you have it! However you got there, you should now have reached the end of the current demo :3 I'll do my best to update this guide further with more information as the game is updated. Once more characters are added, it might be easier if I make separate walkthrough documents for each different character, and possibly even separate ones for a character's Sweet & Sinister sides just to make sure things don't get too messy x3