Mr; master; boy
Ms; miss
「君」 has other readings with different meanings.
田中君は風邪をひいて今日は休みです。
Tanaka is absent today because he caught a cold.
山田君、すみませんがちょっと来て。
Yamada, excuse me, but could you come here for a moment?
分からないことは佐藤君に聞いてみよう。
Let's ask Sato if we don't understand something.