
Holy crap, I was actually right about who the White Room assassin is!
Tayuka Yagami, I KNEW IT WAS YOU, BITCH!
Just got done reading vol. 4.5 of the second year. Haven't seen season 4 of the anime yet. Don't know what happens next. Obviously no way to prove that I'd been suspecting him since vol. 1 and everything in the following volumes just added more strength to my suspicions, but this isn't me trying to flaunt "Ooh, look how smart I am!" or anything, this was just me having fun with the mystery, especially since I heavily applied the "Nerds at Nite" method from his analysis of the Scream movies (despite this not really being a murder mystery) and it actually worked.
Rule #1: You gotta have suspects
Rule #2: You have to have clues
Rule #3: You have to have motivation
Rule #4: You have to have opportunity
Rule #5: You have to have red herrings
Let's go through all the clues the story gave us to potentially uncover who was sent from the White Room to get Ayanokoji expelled, i.e. The White Room Assassin (since there are two White Room students and I could be referring to either of them if I just said that).
First, we know the assassin can't be a second or third year student. Even though Tsukishiro hypothetically has every reason to lie to Ayanokoji (case in point, he had lied to him about there only being one person sent from the White Room) and thus we can't just take him at his word that they were put into the first year students, the fact is that the Advanced Nurturing High School has each grade level closed off until graduation. A new student cannot just transfer in, and even if Tsukishiro or Ayanokoji's father could somehow bend the rules enough to force them in anyway it wouldn't do them any good since someone new joining the second or third year when that has never happened before would immediately make it obvious that they are the assassin.
It is possible that they could be a new teacher. However the point of putting someone from the White Room in as a student was so that they could attack at Ayanokoji in ways Tsukishiro could not as the Acting Director and an adult. Therefore as far as the person Ayanokoji needs to be on the lookout for the most likely possibility is that they are indeed a first year student, so we can eliminate any new characters we get introduced to outside of the first years as our suspects.
Second, while more of a guideline than a hard rule, a good mystery will typically not introduce the culprit late into the story. It doesn't matter how many clues the audience gathers if there's no one for said clues to even point to yet. The longer the story goes on before the reveal, the later a character is introduced into the story, the less likely it is that they will be the culprit. For most mystery movies this means the culprit is typically someone who was introduced in act 1, and likewise for this series the culprit was likely one of the first years introduced in vol. 1 and given some level of notable screen presence.
This means our primary suspects are Nanase, Hosen, Yagami, Amasawa, Utomiya, and Tsubaki.
Third, Ayankoji's narration makes a direct remark that Akito having heard of Hosen back when he was in middle school does not clear Hosen of being from the White Room. They've been preparing to send someone into the school for months, which means they've had time to prepare, research, and teach the assassin whatever they feel will be needed to make them blend in. It's very much within the realm of possibility for the assassin to be assuming the identity of an existing person, especially within a school as closed off to the outside world as much as ANHS is, meaning there is no way for anyone to confirm whether they are or are not who they say they are.
So our suspect can be someone who supposedly lived outside of the White Room, including those who have been heard of by a character we can trust or that even supposedly went to school with a character we can trust to have no affiliation with the White Room. By the same coin this is the story potentially giving us a hint that the assassin is a character who has made their background known through another character, such as Yagami supposedly having gone to the same middle school as Kushida and being one of her admirers from a lower grade.
Fourth, there is what Amasawa later tells Ayanokoji about how the 5th generation within the White Room had a greater focus on social and communication skills, meaning they are better at masking, emoting, and generally interacting with others. So our culprit more than likely does not have a personality like Ayanokoji's. They are not someone who fades into the background or tries to hide and not stand out, nor are they someone who has trouble in social situations. This makes Tsubaki in particular a less likely suspect, even after she starts showing her true skills as Class 1-C's secret leader for the Island Survival exam, or arguably especially after, given how much of a similar role she is fulfilling to Ayanokoji in his dynamic with Horikita.
Fifth, the first chapter of the second volume gives us a small look into the assassin's mind through their narration, thus we know for certain that they are someone who hates Ayanokoji, having resented him because of how they were compared to him all their life, and wants to prove that they are better than him, to the point they may be willing to even kill him if it'll allow them to be deemed as the White Room's success over him.
This means that even before we get direct confirmation that it's not them we can rule out Nanase and Amasawa because of their respective first chapter narrations in vols. 3 and 4. Nanase wonders if Ayanokoji is evil and wants to send him back to the White Room to prevent suffering, and Amasawa greatly admires Ayanokoji while also having no desire of her own to return to the White Room. They do not fit with the motivations we know the assassin to have. This also potentially rules out Hosen, who only became obsessed with fighting Ayanokoji after he failed to get him expelled and Ayanokoji surprised him with his strength and toughness.
Sixth, the assassin disobeyed Tsukishiro during the Written Exam where the first years pair up with the second years, not getting Ayanokoji expelled through that exam like Tsukishiro had wanted and getting the whole thing over with before May. So they are not one of the people who tried to partner up with Ayanokoji, which further lowers the odds of it being Tsubaki or Utomiya, or even of it being someone who directly approached him like Nanase or Amasawa.
Seventh, the note that was left for Horikita to warn her about Ayanokoji's potential expulsion in I2 was described by her as having beautiful handwriting. In volume 2 Kushida directly remarked on how good Yagami's handwriting is when he was meeting with her and Ayanokoji to warn them about the bounty on his head, with his explanation being that his grandfather was a calligraphy teacher who taught him. While this is another potential hint that the story wants us to keep in mind it doesn't immediately confirm that he's the assassin. However, odds are good that the note was left by someone from the White Room because Amasawa recognized the handwriting, thus why she tore up the note. And since Yagami is on the student council we know he wasn't one of the 66% of students participating in the Treasure Hunt game on the ship, meaning we know he is someone whose handwriting Horikita couldn't check.
Finally, if the assassin is refusing to work with Tsukishiro and wants to defeat Ayanokoji on their own terms, then they have to be someone with connections of their own to manipulate people, pull strings, and know what's going on around them since they cannot rely on anything the school has set up for them. Someone who has enough trust from others and can get others to work with them, be it because they trust them, such as the first years agreeing with a certain person's plan to work together to earn first place in the Survival Exam, or because they believe that they'd know what they're talking about, such as Kushida being told by a certain someone where Ayanokoji and Nanase are having their fight or Tsubaki and Utomiya integrating a certain someone to get the secrets of Kushida's past. Someone who is involved enough with everyone else to push things in the direction they want them to go.
None of these factors give us a 100% guarantee that it's Yagami who is the White Room student after Ayanokoji, but compared to all other possible suspects the most lines up with him. Everybody else has something that rules them out or that has it make less and less sense that it would be them, in the end leaving behind just one remaining strong suspect: Tayuka Yagami.
But what do you think? Did I miss any clues? Are there big holes in my logic that I'm just not seeing? Did I COMPLETELY misread that chapter between Yagami and Amasawa and am currently eating my own foot because he isn't the White Room assassin? Please, let me know.