GlaDOS from the Portal series in human form (termed gijinka, I think it's spelled). Hey, it's much better than a potato form, right?
Unfortunately, once again, the cake was indeed a lie.
I didn't notice until now, but there's a gijinka turret in the chair next to her.
There were also 2 Chells walking around, and they were with GlaDOS at one point. One had a large Weighted Companion Cube and another had a full-size Aperture Science Handheld Portal Device.