They're very alike, aren't they? The first time that the Dark Magician Girl is mentioned in this season, it is when Yugi Mutou met Mana. The girl didn't either know what was this person, but going on with the story, she will find out that the Dark Magician Girl is her ka, and when she'll become a powerful magician, she will be able to summon her.
They are a perfect couple, in fact the power of the Dark Magician Girl depends from her and during all the battles, she menages to do her best.