Everybody knows who this guy is: it's Master Chief, Spartan 117. It's my interpretation of him as there were certain things I didn't necessarily like on the original design. I changed the design for Cortana cause it seemed strange to me that she had virtual hair, but was completely naked. So why not give her virtual clothes? (I know the AI from Halo Wars had clothes lol)
AIs chose their own appearance. Every AI looks different. Déjà wore a toga and carried a cuneiform tablet, for instance. So Cortana chooses to look the way she looks.