Holy shit, I totally didn't expect that ending. Honestly I was so sucked in with the atmosphere you presented and the stillness of the entire game, I was so focused on being prepared for a jumpscare that I was blindsided at the ending. You really played well with our assumptions and understanding of the yandere trope, and you made the characters really well as in they acted the "role" we automatically assign them with. Which made the ending so much more impactful. SPOILERS BELOW!
When Siren was introduced, I genuinely thought he was the yandere, but it stopped as we kept progressing through the game and as we learned more about Siren, he felt so much normal and kind. I adored chatting with him and how it feels like he's like a spark of sunshine in this gloomy world (I love how the bus represents what I think Nao sees the world) and not only that, but the way Siren consistently gives effort to be with Nao even as a friend really touched me and makes me, in some ways, understand why Nao had to leave or else he would commit something far worse. Maybe even dull down Siren before he began to fully bloom into himself. Now, I'm going to go with a theory about Nao because good lord, that ending and the 180 of his personality hit me hard!
I'm not even sure if Nao is human or if he is a human host for a much darker entity. It seems as though he was human first before something attached to him and he just had to accept it as part of him. The entity itself seems to be some sort of dream like demon (I'm using this term very loosely) or that it can manipulate its prey's sense of reality and warp it (given the state of the bus). The both of them may have found a home in the forest (as evident in the greenery in the bus and the tree branches that flash through the screen) and could explain why Nao himself doesn't own a computer and a rather old phone. The reason why he could even be in net security is either he lied since Siren mentioned he was smart or that he uses his job as a means to spread the entity throughout the population as a sort of evil chain mail thing. At least, that's what I think it is.
What intrigues me is how Nao was able to play as this skittish person, but maybe he wasn't "acting". Maybe that's genuinely how he acts in front of other people, especially to those he cares about (Siren), but towards the entity he's more bold and authoritative, as if he knows he is the gateway the entity needs to cross between its reality and this reality which is why Nao is able to boss the entity than possible.
I just really loved how I was so blindsided towards Nao's true nature, he played it really well, able to earn sympathy from not only me but to Siren who, while sadly he said the "right" words as a friend, effectively doomed him and Sheila due to Nao's jealous/envious rage.
The atmosphere was so well done, I really love it! I'm just glad I was able to play one of your scary games without being a wuss because having to move around absolutely terrifies me ahahahah!