Flag module can be used to mark users in a Drupal project. Flags can be activated globally (where the flag status is the same for everyone) or personally where each user can control their own flag preference for another user.
The most common use of user Flags is to create lists of Followers. Users can add others to their Interests list, and have Followers who are interested in what they create on the site.
This article shows how to configure a Follow flag and create Views lists of Interests and Followers to be added to user Profile pages.