According to EA France, Finn and Captain Phasma will join Star Wars Battlefront II on December 13th and can be unlocked for free during the "The Last Jedi" Season.
Unlike all other heroes in the game, the two newcomers will be available for all who play the game before the season ends, reverting back to their 40,000 credits value upon the conclusion of Episode VIII's promotional period.
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"The Last Jedi" Season will last for four weeks, and is expected to be followed by another DLC period. According to EA before the launch, all future DLC for Battlefront II will be free to all players.