@O-Takuni With the way the story is progressing I think they're presenting Shizu as a realistic non-romantic tsundere, which is... a type of person not a lot of people would like in real life. At least that's a reason I see why she isn't likeable.
Maybe to Narumi she is a tsundere, but seriously to Chiho, that's bullying . She never cared about Chiho and just purely hated her. Someone's "stupidity and nuisance" is not a good reason to hurl insults at them every day for a year.