～君
～くん · ～kun · JLPT N4 · noun
mister
👦 kun = the junior tag: san for adults, kun for boys and juniors
The kanji 君 on its own reads kimi, the casual word for you, so ～kun carries the same familiar, speaking-downward tone. Teachers and bosses attach it to boys and younger male colleagues; never use it for someone above you.