Well the Brigade members trust him since hes nuetral to their antics with their alliances so it's no wonder these regular high schoolers are smitten with him. It's either him or the ambigous Itsuki who would only get along with...Mugi.
I personally can't find how Kyon is attractive, he seems plain. This type of guys can be found anywhere. But yeah, kinda hard in the anime world where most of the protagonists are the same as Yuuki Rito.
That Kyon seems plain or boring is part of the intention and draw of his character. You're supposed to think he's dull. Then he'll say something snarky, speak philosophically, or reference something extremely esoteric, which is when you realize that he's anything but normal, average, or plain.
His situation alone makes him the most interesting one in SHnY by virtue of being the only alleged normal. That, on top of knowing the various things he's not supposed to at his age, also adds elements of mystery to him.
Lastly, on top of being smart and well read, he's also shown to be witty, easy going, sensible, empathetic, and trustworthy. In all, he's really far ahead of his age in most aspects. And these are all things women like.
In short, Kyon is a bro and to class him with typical, lame harem leads is a travesty.
Even the teacher wants him. And what's even better is Kyon can have whichever cake but the H, and eat it too.
Uh......if I were dating him, there would be nothing else in my life.We must take action against things like this! Please come with me!!Mmh...?Haruon! 19Mio-chan is having an underage sexual relationship, so I should warn her? But such a private matter is none of--You can't say that~!!