How did you even think about Lev? All those years? It didn't even cross my mind.
Several reasons. This is going to be long, so I'll add a cut:
Near the release of Lostbelt 5, I started to get suck into the Animusphere conspiracy theory which I always supported from the very moment the PV of part 2 dropped, including that the "Alien God" was going to be Beast VII. Let me tell you, that wasn't easy when the 80% of the fandom kept pushing Sefar, Velber, or the outer gods. To me, the bleaching needed to come from the future into the present, like part 1 was a threat from the past for parallelism. And its source was Chaldeas' texture because of the projected 100 years, and the weird way the Bluebook's sequences never had a single year. This made me sucked into that crossover manga with Sion (Melty Blood Backalley Nightmare) because of Olga being mysterious, powerful, and central to the story. During it, I also started noticing Lev was important to that plot. Lev was a character who was glossed over in the fandom. I also admitted to doing that before this, but I started to notice him and how Nasu was expanding him, with or without Olga. Of course, I knew of his debut novel and read it, but that didn't take place in Fate universe to pay attention to him.
That was a seed in my mind, but Chaldean teased around being Romani or Goetia (King of Humans, I should clarify, not a demon god survivor) were likely suspicions, especially the latter, which was a simple option, widely accepted. So seemed to await his identity reveal in Atlantis or Olympus. A breaking point in building him up. But…that didn't happen. That's how I knew the guy couldn't be Goetia (KOH). He wasn't Roman, but he couldn't be Goetia either. Why? Because Nasu was presenting him as a big enigma to be solved, whose identity is meant to confuse the reader, to continue obscuring it. It would make no sense if the answer to his Mystery Man was someone everyone guessed from the very start. Why bother to make "Goetia" a mystery?
The other reason was his characterization. All fans knew he wasn't Solomon, but I also knew that guy couldn't be Goetia (KOH). Goetia, the sentient spell that served as a control server and became human, was never 'tsundere.' Stern, arrogant, serious? Yes, but tsundere and dishonest? Never. He was actually very open about his offer to help Mash (even sought her out twice), and described his feelings for Guda in the last scene together. He could be slow and misunderstand things, but he was never pompously denying his feelings. There's a fanon he would be tsundere, which is cute, but that's not how Nasu wrote him. Flauros, on the other hand, despite being the one who wanted to save Mash (and Goetia was just offering on his behalf) never admitted it to her face. He never admitted he was slighted and wounded by Roman's lack of trust (ironic as it is). He is, actually, hmm, emotionally dishonest. The idea it could be Lev emerged in my mind during Atlantis, not just because of the pile of Lev content Nasu pushed, and this personality match (and because I was certain it wasn't Goetia), but due to two key scenes: the one with him describing Chaldea's struggle in the Lostbelts (a parallel of Flauros describing the same with the Singularities), and his entrance…he lands to save Chaldea from the same fate Lev had thrown to the Crypters as an act of balance. Lev was even mentioned in this conversation, and Kirschtaria was familiar with him.
The following year, with Lev in the OP of part 2 with Romani just made me work the courage to propose that theory, flawed, and under works, and revisions.
I was mostly 99% sure it wasn't going to be Goetia (KoH), the more likely contenders were: Chaldeas/AU Solomon, or Lev. You either push his identity reveal because it could be linked to the plot (Chaldeas Solomon-Fuyuki, possibility) or because he's a twist in itself (Lev).
In hindsight, without biases, you can see the trajectory of the Chaldean's introduction since 2017-2018: Lostroom happens where Lev is a main character, he leaves the Room (as does Olga), and then someone grabs Solomon's ring from the Throne. Immediately after, we have a PV with Sugita, who claims to be Solomon. Without fan attachment to KOH Goetia, it should be clear it's Flauros, right? He was also part of "King Solomon." The connection between Olga and the Priestess was immediate, and she was in the exact same position as Lev. Olga appears as a focal character, leaves the Lostroom, and a mysterious woman is watching (who later appears in the game when Olga is discussed), fans immediately agree she's the robotic voice who chants the animusphere spell in the same pv that aired after Lostroom. Yet Lev, another of the main focuses in Lostroom, was ignored to conclude that it was obviously KOH Goetia.
The way Lev got ignored while Olga wasn't just how fan bias completely twists something that was almost under our very noses since late 2017-2018 (the new years stream).