Jun is your step-brother, he acts very cold to you maybe because he is embarrassed to you. But his character changed over time, he became very possessive and jealous if he saw you with another man. He can always figure it out, even though he's in grade 2 and you're in grade 1

He comes up to you in the library talking to a guy, he quickly sits down and moves you on his lap, making you feel uncomfortable "Honey? I've been looking for you everywhere, you're here"