I think
@UdenEmpati is right. He must be the first boy Sayo likes and she's running at 200% going after him. Also, I don't think she's a yandere. As for Matsuura, he must not have interacted much with girls (and his few encounters have probably ended in confusion or disaster), hence his mistrust and complete misinterpretation of the situation.
Thank you for the translation!
I'm actually interested to learn if there's a trigger for the reason Matsuura feels like he's an outsider with a unique grasp of the human condition (at least going by how he frames his interactions with classmates in his narrating in the first chapter).
Given he sorta acts like he's got a soul-read on people and knows what they're really thinking, it almost reads like some sort of chuuni-adjacent "fantasy", like a sensitive kid feeling like he can read his classmates' minds because he's particularly attuned to body language or something, but lacks the actual knowledge in correctly sussing out what's really going on. (Totally not speaking from personal experience or anything, nope nope not at all)
It
could be that he had some less-than-great interactions in middle school that have caused him to turn inward and become insular and aloof toward others, but his self-proclaimed "I see the bigger picture" attitude coupled with his clearly-incorrect read on the FL's demeanor and true intent, sorta read like he's just a bit fantasy-bent and thinks he's got some special trait that exists in his own brain, and he's really just your typical goofy kid whose head is in the clouds a bit, and just lacks experience in this particular situation and only has manga and third-hand experience to go on.
Though, given he
specifically thinks she's a man-eater and player who's only seeking to screw with him, your idea might very well hold water, and/or it ends up being a mix of the two.
Either way, it's honestly kind of endearing, especially since we get
none of her internal thought process and are wholly reliant on what we see her doing & saying around him to get any idea as to how off-base his interpretation really is. Makes me wonder how many chapters we'll go before he figures it out (or she loses patience and speaks up about liking him).