How to use flags to pass variables to users?
Consider using the Profile 2 module. Excerpt from its project page:


  Designed to be the successor of the core profile module, which is deprecated for Drupal 7. In contrast to the deprecated module this module provides a new, fieldable 'profile' entity - leverage the power of fields.


That project page contains a lot more interesting info, such as these Features which might help for addressing this question:


  
  Multiple profile types may be created via the UI (e.g. a general profile + a customer profile), whereas the module provides separated permissions for those.
  Optionally, profile forms are shown during user account registration.
  Similar to the deprecated core profile module, fields may be configured to be private - thus visible only to the profile owner and administrators.
  By default the UI behaves similar to the deprecated core profile module, thus showing each profile type in each own tab below user//edit. Via the included "Profile pages" module profiles can be viewed and edited at their own page and get their own menu link, e.g. "My profile".
  Thanks to the foundational entity API the module integrates well with Rules as well as Features - thus profile types and the associated fields can be exported into feature modules. Once the Entity tokens module (part of the Entity API) is turned on, there are also token replacements available.
  You may use Views and/or the Search API to provide listings or searches for profiles.
  


Refer to its community documentation also for more details about this module.