I've only played the demo so I cannot say that much, but I'll try my best.
First off, I love the concept and premise. The idea of watching someone's personality develop from nothing in a simulated world and what this means for their existence is very interesting to me, and it can say a lot about the nature of humanity and what makes us who we are.
The aesthetics are great too! From the art to the UI, everything conveys the surreal and simulated feeling the game is going for. The music supports this feeling too. You always get the feeling that something is off or fake in some way, and given some of the things that are revealed later in the demo, this is very well done!
Some nitpicks for me are that the voice acting (which in itself is good) is not affected by the voice volume slider and instead responds to the sound effects. If you're like me and you read faster than the voice over, then you'll have to choose between constantly interrupting the voices or not having any sound effects at all. A second minor gripe I have is that the puzzles are a little too easy for my liking and can be easily cheated even if they weren't. Adding something like a timer could help with this, but, on the other hand, I can also imagine them being easy is intentional since it's still early on in the story.
As for the writing, I have two suggestions for improvement.
First, and most importantly, emotions and reactions are not described very thoroughly. The game opens with 159733's motivation of not letting anyone feel lonely because that is what they had to go through, but this feeling is only stated and not fully described. Specifically, they describe feeling 'unloved, but not necessarily hated', but this remains rather vague because it is not compared to anything tangible and not supported by an example. While it is clear from a logical point of view, it does not involve the reader as much as it could. It's the difference between stating that a character was neglected as child and describing how their parents were always at work, that they would never pay attention while playing with them and would always claim to be busy when the kid wanted to talk about something. One is a factual statement while the other takes the reader through the experience and lets them fully understand what it is like.
It also happens later on when Maephkastel's voice is described as containing 'an amount of emotion' never heard before. This could mean anything and the intensity is also left a little vague. It's not clear if he's sad, angry, nervous or perhaps something else, and the reader doesn't get a clear idea of what this amount of emotion sounds like. Is his voice raspy or surprisingly quiet? Are there pauses or stutters? When? What could this mean? Again, you should lead the reader through the experience and let them feel what they're supposed to feel. Similarly, 159733's reaction to the reveal of Maephkastel's age (won't say more because of spoilers, but I think you know which part I'm referring to) seems to almost be nonexistent since they immediately ask regular questions afterwards. This is supposed to shatter everything they know, but I didn't see that in their reaction.
When people mention 'show, don't tell', (although I personally don't really like the phrase) this is typically what they mean. It can be argued that emotions are supposed to feel more subdued at this point in the story given that they are still developing, but then this should also be described in some way. More importantly, the motivation of both characters seems at least partially emotional so that implies they are important here. The best piece of advice I can give here is to imagine what certain emotions feel like physically and what emotional reactions should look like on a person's face or body. You don't have to do this for every single reaction as that would slow the pacing down to a crawl, but for major reveals and 159733's introduction, it could help a lot to spend some more time establishing what their experience feels like.
Second, although it is related to the first point, 159733's insistence on talking to Maephkastel is contradicted by their earlier shyness. They have to really convince themselves to interact with the other embryos and remind themselves that they are not a bother to them, yet will keep going after Maephkastel despite literally being told to go away and leave them alone. I understand logically that 159733 is trying to not let anyone feellonely, but the ease with which they seem to do it makes the earlier nervousness feel unimportant. If 159733 were showed to have doubts all throughout their conversation with Maephkastel or not described as nervous in the first place, then this would not be a problem, but as it is now it seems that her personality does a 180 turn pretty quickly. It could also work to (very quickly) describe 159733's interactions with the other embryos and then shift into the introduction to Maephkastel while making very clear that they are literally the only one not socializing. Then you can emphasize 159733's mission while still describing their reluctance and nerves when approaching Maephkastel. This could be an important kind of growth for 159733 but it's glossed over pretty quickly.
While writing it's important to consider what a character is thinking and feeling in any given moment and how they would act because of it. That obviously makes it difficult to get to the scene you're trying to get to, but that is again a moment where you should stop and think about how a character would realistically act and how you can change that.
Overall, don't be discouraged! I know it might seem like a lot of negative feedback but that's only because I'm trying to be specific and thorough enough that you understand what I'm trying to say. I still think the fundamentals of the story are solid and that it's headed in an interesting direction, but the writing style could be more immersive at certain points. The reason we read stories, after all, is not to get a logical explanation of what happens but to empathise with the characters on the screen/page and through that learn things about ourselves. And don't worry if you struggle with some of this; it's one of the most difficult parts of writing and almost everyone struggles with finding the right balance, but it's also one of the most rewarding for both sides.
So, in short, I can definitely see the potential and the concept is really interesting. I'll check out the full game for sure and I hope I've given you something useful. Keep writing!