Here's the list of first introduction in Metal Slug series by faction.

Regular Army

Rebel Army

  • First main antagonist that appears as a boss: Donald Morden
  • First major antagonist appears as a sub-boss Allen O'Neil
  • First female Rebel Army member: Abigail
  • First experimental soldier: Vita
  • First scientist to work for the Rebel army: Navy
  • First character to have a personal vehicle: Rapid
  • First conscripted infantry: Huracan