Okay, so I have a clearer picture on our character's inferiority complexes. (TW: something of a trauma dump ahead.)
Both have a similar source: A parental figure put exceedingly high expectations on them, making anything they do that isn't PERFECT feel like failure.
Now Narumi's abusive father is out of his life, but he's spent so long in this mindset, that nothing he does ever feels good enough. Combined with the fact that his mother remarried (it's very common to feel like an outsider in mixed families), and his sister both accels and goes put of her way to include him only makes him feel guilty and I significant by comparison.
Kazemiya, on the other hand, still lives with her abuse. Her mother has seemingly given up on her, and takes no interest in anything she does. In reality, I wouldn't be surprised if her mother thought that she was doing something for her: her version of being less restrictive and demanding of her, and letting her make her own choices. But the way she's gone about it, and the way she speaks to and carries herself around Kazemiya puts an air of contempt over her.
Personally, I've experienced similar forms of parental abuse. My Dad was the kind of person who saw things he didn't like in himself and took them out on me. Nothing I ever did was good enough, every accomplishment was downplayed, and I was always expected to know what he wanted well in advance, or there would be hell to pay. I would work myself until I burned out and my panic attacks where more proof that I wasn't good enough.
It's been years now since he died, and even with lots of therapy, I struggle to feel any kind of accomplishment or success in any aspect of my life. People want to say, "You were a kid, just get over it", but it's those childhood years that influence your personality and brain chemistry the most.
Trauma and abuse can take decades to overcome. Even with lots of love and support, the trauma can make the victim unable to even FEEL it.