Jaster Mereel was the Mandalore, or Mandalorian leader, of the True Mandalorians. He is also the man who took in and raised Jango Fett after his family had been slaughtered at the hands of Mereel and his groups sworn enemies, The Death Watch. Following Mereel's death thanks to the betrayal of one of his own, Montross, Jango took over as mandalore of the group.
Jaster Mereel is also the assumed identity that Boba went under while living on Concord Dawn as a Journeyman Protector.
The name first appeared in the Andy Mangels-authored "Star Wars: Essential Guide to Characters."