So while I was downloading CC one day, I came across the hair style of the protagonist of Yandere Simulator. While I've not played that alpha in awhile, I recall the protagonist doesn't feel emotions, usually faking them. Which made me think of the potential of having such a trait be an option in The Sims 4. Having looked it up, said disorder is called Alexithymia.

Now, what I'm thinking, is:

1. The sim is completely unaffected by moods that aren't Asleep, always feeling Fine.
2. Sims with the Alexithymia trait would have a new self option, Fake Emotions, which'd force a unique moodlet onto them that has them feel that one emotion.
3. The emotion is always the most basic, and cannot be higher. I.E., no Very Focused, Very Confidant, ect.
4. The trait should be incapable of being discovered by other sims from Get To Know. Instead, the afflicted sim should be able to confess to having it, or somehow be caught faking their emotions.

I honestly doubt a trait like this is even feasible, but hey, it'd be pretty cool if it could be done. In any case, it's just an idea.

I think it's doable. From what I understood, it would be pretty easy even. You can PM me if you want to discuss it further.

As someone who likely has alexithymia, this is drawing on a couple of negative stereotypes about the disorder. One: That we're emotionless. This just isn't true. We feel emotions just fine, we just can't put a name to them. I can feel upset, but I can't tell the difference between angry, sad, or scared for example. On the flip side, I can feel good, but I can't tell the difference between the different variations of that. To add to this, you've also compared what Ayano experiences to alexithymia, which in a more subtle way implies that we do not feel empathy (Ayano wouldn't be able to murder people so callously if she had empathy.). We feel empathy just fine, we just don't feel it the same way people without the disorder feel it. What Ayano experiences is NOT alexithymia. There isn't a word for what she experiences because it doesn't exist.

When it comes to official diagnoses on MTS, it is stated that you should attempt to be true to it, and while your trait ideas are good, they aren't indicative of Alexithymia, so I can't see it being accepted if called that. They don't fake emotions, they are unable to identify them and their differences. If you were to change it to have the real traits, it probably would be okay, but I think it would be extremely difficult to achieve. For your particular trait ideas, I would call it something else and not mention Alexithymia at all. I think what you're referring to is more closely related to Anhedonia.

I found this which seems to fit what your ideas are:
"The symptoms of anhedonia depend on the type. Symptoms of social anhedonia may include:

Very few or no relationships at all
Complete withdrawal from existing social relationships
A diminished capacity to express emotions
Fake emotions in social situations
Preference for being alone
When a person suffers from physical anhedonia, they may experience:

A complete loss of libido or interest in sexual interactions
Frequent illness or other physical health issues
A person with anhedonia may miss out on big life events. By not maintaining their social connections, people with anhedonia may have strained relationships and even a loss of social connections."

--just noticed while the last comment is recent, this post is fairly old, so take my suggestions with a grain of salt

What she (Ayano) experiences CAN be linked to brain damage.

There was a woman that got in a car crash and before the car crash she could feel emotions just fine but after the car crashed her brain was so messed up that it shut her emotions down completely now she did do all of her motherly duties and overtime her emotions did come back meaning her brain healed but otherwise she said she felt completely numb