Junior; someone of lower seniority (at school, work, club, etc.).
後輩に仕事のやり方を教えた。
I taught the junior how to do the job.
会社の後輩とランチに行った。
I went to lunch with a junior from the company.
部活動で後輩を指導するのは大切だ。
It's important to guide juniors in club activities.