When my younger brother gets horny he TAKES a bow like Cupid and shoots you and you DIE.
He's passionate but unfortunately not gifted with a sense of poetic metaphor.
someone's younger brother (say if you're talking to your friend's younger brother)
siblings (not just brothers)
student (more formal, ususally used with the name of the institution: Harvard弟子、 Waseda 弟子）
弟子 学生 生徒 教徒