If you recall, Nintendo/Koei Tecmo accidentally let slip two (or three) new playable characters from Awakening, so the surprise has been slightly ruined. That said, there is one more character revealed who’s not from the aforementioned slip up.
Warning: There are some story spoilers for Awakening.
As predicted, joining Chrom are Lissa, Robin and Masked Marth/Lucina. In addition, Frederick–Chrom and Lissa’s faithful guardian–will be accompanying them during their adventures.
From the footage shown, Lissa and Frederick wield axes, Robin wields magic and Lucina is another sword-wielding hero. Despite appearing as a Cleric, Lissa’s choice of weapon is clearly based off her promoted War Cleric.
Another neat touch is that Chrom, Lissa and Frederick are summoned by the Shield of Flames (presumably this game’s Fire Emblem) during the beginning of Awakening (or perhaps even before).
This means Chrom and company have yet to meet Robin, nor have they met Masked Marth and discovered the truth behind “his” identity. Therefore potentially allowing for a bizarre and dimension-crossing retelling of the major events of Awakening.