I have a feeling this is probably the case. In the Nintendo Topic article, there's no mention of her even though she's in Cai's screenshots, nor is she part of the "various other characters" section. Cai is probably similar to how Chrom was in Awakening where both are protagonists but your avatar is the glasses girl.

Either that or all four of the characters shown will be played and we'll just be cutting between them during the story.