Back to the fundamentals of their relationship, I think it's really interesting how Kazutoshi so easily decided to isolate himself and therefore bring Ken down with him because he had already done that beforehand anyway? When it comes to Kazutoshi doing stuff like that, it's obviously in favor of self-sabotage, but when you look at his terminal he had nobody to bring down with him. Which leads to the event of the killing game.

-> Kazutoshi is taken

-> Kazutoshi meets Ken

-> His behavior is fed into

-> He ends up isolating himself and Ken further

Every time he's in the series, Kazutoshi is usually seen trying to find a 'quieter' place, even if there's only maybe 2-3 people in that room. Sure, you could always take it as him simply preferring the quiet, but when it happens with just a single person in the room, you kind of start to question why. Clearly, Kazutoshi's used to being alone, there's not much debate for that. When he meets Ken, though, it immediately shifts for him. Instead of him isolating himself from Ken in favor of being alone, he starts to isolate them, which somehow manages to have a bigger impact on Ken than Kazutoshi himself.

Kazutoshi ends up thinking of themselves as one thing rather than individuals with their own lives. Honestly, I think it's actually something of Kazutoshi's fault for Ken's lack of merging into the group. Kazutoshi doesn't stop himself from making connections with others (which likely also comes more naturally to him, it's not like he's completely unsociable based on his student reveal unlike Ken who has clear agoraphobia and genuinely holes himself up in his room and only ever goes out for quiz shows and when Mao wants him to.)

But onto the original point, Kazutoshi ends up making more connections than Ken does, like Hayashi and Tamba. But who does Ken have besides Kazutoshi? Because of Kazutoshi's constant desire to be what he believes to be alone and isolated, Ken ends up taking some of that burden since he has nobody else.

With Kazutoshi seeing himself and Ken as people who are meant to be together (which, codependency, obviously), instead of people who want to be together, that kind of puts up the thing of being entitled to the other one's affection. Since Ken is also used to being alone, and sees Kazutoshi as someone to rely on because of how shunned and alone he's been his entire life (also because he's extremely sheltered), he clings onto Kazutoshi instantly. We've already talked lots about how they feel about it, and how it affects them, but Kazutoshi is at so much fault for accidentally pushing Ken post-T3.

That wasn't out of malicious intent, obviously, but it was definitely out of habit and his own unhealthy behaviors. Kazutoshi made a 'place' for himself and Ken, and pushed everyone away, which stopped Ken from making any other friends besides Kazutoshi. I don't think it was malicious at first. Because, he is extremely self-aware in the wrong ways, where he ends up wanting to benefit himself because he's never gained anything positive -> "What's the point of caring if it's not for myself?/Why should I not benefit myself if I'll never get anything at all?"

I'm sure that at first, he finds it just amusing and doesn't pay much attention. It starts off like a 'I'm going to isolate myself because that's how it usually goes,' then Ken comes into the picture and it immediately goes to, 'We're going to be isolated together because that's how it's meant to go and he doesn't think it's bad, he actually kind of likes it too.'

Kazutoshi ends up enabling Ken's Me VS Them behavior, and also his agoraphobic tendencies because of his self-sabotaging.

Funny thing this happened while I was freaking out over 'not liking them enough' which. Liar <- wow guys it does something to you.