Need a little extra firepower? Tap the amiibo symbol in the top-left corner of the Wii U GamePad controller while setting up your defenses, then scan a Fox or Falco amiibo™ figure. Once the team's set, you can call in the cavalry by tapping the Arwing symbol during the mission.
Note: there are certain missions where this feature can't be used. In addition, you can only use this feature once per day, per compatible amiibo character.