In Japan, it is common for people who are close to each other even if they are years older to speak without calling them away or using honorifics.That's why he doesn't call me Hyun when he speaks Japanese.
In Japan put "kun" after name or use nickname are friendly and soft expression, so sometimes used for elder. (a case of between close young men) e.g. Yoshi-kun, Masshii Normally younger don't call elder by only first name, It's ok depending on the situation or a very close friendship like Haruto and Asahi. Yoshi and Mashiho are the same school grade in japan, so they call by first name each other… I guess.
