So it feels like the early relationship is a point of contention, lately. Takumi made this promise to Karua and obviously was happy to be with her, but also started to get kind of standoffish and weird with her even while he was still a gradeschool child. Why is this?
Slightly broken up caps, but the points here suggest that Takumi had established a sort of 'hide and seek' relationship with her - whether it was on his own, or Karua's mum's suggestion (that part's a bit more vague in the JP) - which is on-the-nose for 'chaser and chased'.
(He's really passionate about it, too- he loves being her hero.)
This leads into Takumi's fixation on keeping the status quo- he has One Job(tm) and everything else feels superfluous. Not that he would know that this isn't a healthy way for a relationship to be, his ages here are like five-six, to maybe ten years old. Still very young.
From a child's perspective, that's a lot of time to be caught in a cycle of lose and find, protect, etc- it's likely going to wear on a kid after a while, doubly if said kid doesn't allow himself to think of much else. This would probably account for a lot of Takumi's mounting irritation, especially when Karua wants to bring their dynamic to the classroom.
Not helping the matter is that while they're still kids - maybe slightly older, though Karua looks much the same here - Takumi's mother gets involved, bringing Karua in to their home without his prior knowledge. Whether this becomes a pattern or not is hard to say, but given that we also see similar happening in the opening sequence, it would account for why Takumi seems to be stuck in the loop to the point of reflexive irritation with Karua's presence as a late teenager.
(Karua takes on more of a supporting role in this scene, at the least, though Takumi is still his usual annoyed self. Figures)
What I find curious here though, is Takumi will find at least some interest in 'new to him' things Karua talks about- it's just that he winds up bringing it back to 'potential dangerous thing, assess for safety' pattern of thinking that he displayed in the classroom scene. In this case he questions, rather than deflects and denies- like there's some connecting tissue there between them, just that there's too much baggage in the way of him really investing himself in it.
Maybe he wishes he could be that passionate child self, so he shares in Karua's 'vintage films' interest- symbolised by his hoodie