Simply put, drugs are about the highs and lows. When you're "horny" or "hungry" in your day to day, your brain tells you where to find "relief". For hungry it's food. Humans overeat, most of Americans are obese. Because we can't control ourselves. That's the "easy" side of addiction. For horny, we do crazy stupid, sometimes deadly things to feel orgasm. Yet, for both of these things, we feel an immense sense of regret afterwards (usually unless you had nice sex). Your brain satisfied that "desire" and you no longer crave it.
This is x1000 for drugs. I can speak from experience of people I know in RL who are addicted to heroin. Heroin is one of the most addictive stuff you can take. If you ask a girl whos an addict to suck your dick, eat your shit, and do whatever to give her a hit, she'll do it. But, after she hits it into her veins, she'll feel a crazy high. She'll close her eyes and feel like she's in heaven. Yet the drug eventually goes away. Your sense of reason comes together. She'll feel disgusted. She'll go home. She'll cry. Her family will support her. And she'll swear to never do it again. She'll be shivering (think being hungry, but 100x worst), can't sleep, can't think about anything BUT heroin. And of course she fucking relapses. Steals her parents money for drugs. When they kick her out, she'll sell her pussy for money for drugs. That my friends, is addiction.
Aya in this game does NOT exhibit the natural progression of an addict. She never has these "refractory" periods where she's utterly grossed out by her actions. She just goes goes goes, sex sex sex. The most unbelievable part for me was on Day 4 of MC being a ghost, fugly man comes to her house, drops the bottle of drugs. She goes into bath, masturbates and cums with it. So satisfies her addiction, she just played with her cunt. Why would she open the door for fugly man again just for him to enter her space? Makes no sense.
Furthermore, if Aya WAS like a real addict (but a strong female girl like we saw in high school), she would have brought a knife or something. Go to the massage parlor where fugly is. Fucking stab / threaten him to give her the drug. And walk out without ever touching his dick again, because ew? Plenty of hot guys out there to bang?
This is all assuming it took a single hit for Aya to even be addicted. It's mostly unlikely for real drugs, since she as a prim and proper girl would know how dangerous it is and do her very best to NOT take that drug and masturbate in the shower.
Other than that, what everyone said I agree with. The scenes are just too abrupt. She fucks everyone and everything. You only get to see it in the CG room. It's a narrative disaster at the end. So much promise.