From Destinypedia, the Destiny wiki
Kadi 55-30 is a Frame Postmaster in the Tower that serves multiple purposes. Kadi stores messages for Guardians, and sometimes rewards for certain actions. For example, Kadi will reward them with a rare material or Glimmer for completing a gold-tier Public Event. The Postmaster will also collect any Engrams a Guardian might have missed in their last mission.
List of appearances