Ah but I think you're mis-characterizing what I said - or at least what I meant. I didn't say she was being depicted as a robot. That term is a bit vague anyway. It's more like this. Her appearance from the neck down is at least reminiscent of a robot look (of advanced type), in the way things appear put together.

That said, perhaps it could indeed make sense for this to be an actual physical construct. It seems to me that her head has a different look to her body. It seems a bit diffuse or soft, with lower contrast than the body - and I also see some horizontal banding on the skin, though I'm not sure if that's actually there or just an artifact of the video encoding (I don't have the game, I'm just looking at YouTube vids). So here's an idea. Perhaps from the neck down we're looking at a constructed physical body (Forerunner/Promethean tech?), which is serving as a vessel to carry her holographic form around - which manifests as her head. Think of the body as her own personal mobility unit, in human form. And as a side thought, perhaps that band she has around the 'collar' could have something to do with the projection of her head.

: I was looking for a clip from that scene to check it again today

I've been looking at this part of mission 14, in 1080p.