Yami Yugi is a spirit that lives inside Yugi.  In the beginning of the series he is quite violent and would do anything to protect Yugi. He is an even better strategist but also pretty competent when it comes to physical feats.

His voice should be deeper than Yugi's  and have alot more confidence and dignity.

 I would like him and Yugi to have the same Voice Actor, but if two people have great voices for each individual character they may be picked.