Student or Learner
How can it be expressed well?— If you want to know that a person has no purpose in life/in their lives, they either copy others or rebel against them.
I find that hard to fix, because of the implicit assumption: if a person is either copying others, or rebelling against them, they have no purpose in their life. It seems a very broad way of identifying people with no purpose to their lives.
In any case, if you're talking about one person, I'd say "a person has no purpose in their life ..." given the gender problem.