This is just how her setting works, in the first few episodes even other students call her KYH
.
My guess is the author will leave it as it is (for now), at the end of episode 5 she told Haru her name, indicating and emphasizing that Haru is a 'special' person to her.
Most likely we will get her name when Haru&KYH start 'officially' dating with KYH asking Haru to call her by her name (continuation from hospital), or we will never know.
Though I hope as well the author will introduce some of KYH's family members.