suffix appended to someone else's family members
「君」 has other readings with different meanings.
you; buddy; pal
Mr; master; boy
父君は元気でいらっしゃいますか。
Is your father well?
若君が城に戻られました。
The young lord has returned to the castle.
姫君の昔話はとても有名ですね。
The story of the princess is famous.