...just remembered, the reason that Takumi is taking the 'dad = protector' role is because Karua's dad shielded her with his body during the
I've had the impression for a while that this childhood recollection was a traumatic memory, but never was this more apparent than when Takumi experiences something that's unquestionably a nightmare - in particular, relating to his failure to protect Karua/Nozomi - and he reacts the same way- waking up screaming, heavy sweating, heart racing. He's not feeling good about either one.
But there's something more to that first memory-
This is very on the nose way of spelling out one of the reflexes of anxiety- the 'Fight' reflex. Funnily enough, it's the very thing that makes Takumi more high-geared for battle over thinking deeply.
When you're almost onto something, but any further thought is so violently unnerving you have to squash it down and run away
And the 'Flight' reflex, causing Takumi to run from things that are making him anxious- exercising in order to decrease the anxiety. Which also feeds into 'preparedness for battle', thus into 'Fight'.
Most likely, he has felt that he is unable to question that role as a protector, since even as a gradeschooler is stuck trying to assess for risk vs reward. Thinking about things that changes his concept of the world feels dangerous- and in a high-adrenaline/cortisol state geared around 'promise to protect someone from dangerous things', makes about as much sense to him as trying to touch a hot stove.
Takumi has mentioned that he 'gets like this a lot' when he has a lot of spiralling thoughts that he can't reconcile, and if you combine with his chronic inability to question his own role, it gives the impression he's been trapped in this state to the point of learned helplessness. Thus when the people around him in the complex didn't validate Karua's curiosity, it likely led him to passively accept the majority.
And when Takumi's by himself, he has no chance of breaking his spirals other than by feeding further into his anxiety.