The leader is a very important role in idol groups. The leader is a liaison between the group and the agency, acting as a link in the chain of command through which instructions, feedback, and admonishments are passed down.
The leader is expected to be responsible for the group and has some authority over the group like a mini-manager. The leader is expected to put the needs of the group first. Members often need to check in with the leader or ask permission to do certain things.
The leader is most often the oldest member. Usually when someone younger is made the leader they have been a trainee with the company the longest or they are a very responsible person with innate leadership abilities and they possess a very obvious star quality.
Most leaders are good at representing the group in interviews, so often idols who talk the most are mistaken for the leader. Some leaders excel in public activities like variety shows and interviews, but all leaders have a much bigger job behind the scenes.
Along with the title of leader, many idols have other prominent roles in the group like main vocal, rapper, or dancer that show off their talents onstage.
When an idol introduces her/himself as the leader, they don’t usually name any other role, because everyone knows that being the leader is a big job.