I'm not too sure I like the idea of a full-height avatar with no face. For one it's a tad creepy and two it takes up a bit more processing power than what is needed. What I was thinking would be something like a mask (such as what Hex uses in the Discworld series) That would float in the air. When not in use it could attach to a wall or something, but when Cortana it invoked it can move to a respectable distance to your side (so not to unnerve by suddenly appearing inches from your face) and position so it is always facing you and generally doesn't move unless you're about to collide with it or you are walking away and it needs to follow you. Thoughts?