I've thought about using nonchalant, insouciant, carefree, blithe, and so on, but they don't really communicate that the character does actually care about what they're doing, they're just pretending not to care. Here's the context if it helps:
"The great king of nothingness and empty space," the officer said, closing her hand slowly around the bit of metal she had been handed.
"The king... Yeah, that's what people call me," Scott confirmed, lowering his brows in a cautious glare. “Though, there’s no way I can be 'great' if I rule over nothing,” he pointed out. It felt redundant to say it now, after he had been ignored so much by Anna whenever he pointed it out, especially to say it to her sister Zara, who apparently felt the same way she did.
“Fair point,” Zara said, following it up with a
???“hmph” as she tossed the ring back in her drawer.
I'm trying to imply that Zara thinks Scott is selling himself short, but that she doesn't want to argue with him about it.