I'm rather partial now to the idea it's Hera. I know D'Anna saw an adult, but what's to say she didn't see an adult version of her? She seemed pretty certain there were only Four in the fleet, but at the same time not too worried about the apparent absence of the Fifth. She knows something no one else does.
Since Hera has no way of making her way to the baseship, I can see D'Anna lying about the number in the fleet, just so the humans aren't hounding her about the who the Fifth is too much.