How about No One. Just because Christ sits at the right diesnt mean theres any one on the left. Its totally symbolic anyhow. They are spirit beings. They dont 'sit' anywhere. Its a way of saying Jesus is in Gods untimate favor, that he is special, because the right is clean and pure while the left unclean. Mary sure isnt up there and i cant imagine why it would be Moses either. The Bible makes ZERO mention of anyone at Gods left. Any mention of someone at Gods left hand comes purely from the Catholic imaginatic, probably declared truth by some pope.
If you seen those cartoons when a person is driven to make a choice, they are driven by the angel and the devil. But there are actually 3 parts of God. 1. Himself 2. Jesus Christ and 3. The Holy Spirit. Now, I'm not a huge Jesus freak but I do believe that he is my lord and savior, and honestly, I don't do much reading of the bible and I haven't been going to church for almost an year now, but it should definitely be the Holy Spirit on your left hand.
The scripture is using a common and very significant metaphor of the ancient near east. The "right hand" of a potentate represented the rulers complete authority and justice. It did not involve a corresponding "left hand" symbol. It essentially means that Jesus is the full instrument of God and in essence being God.