About Dark Magician Girl
Her backstory goes deeper than that, as it is revealed in season 5 of the anime that she is a Ka, a spirit being essentially. In the anime season, she's the Ka of Mana, the apprentice magician to Mahad. Mahad being the court magician to the Pharaoh Atem.
She later shows up in Season 4 to aid Yugi and the Pharaoh in dealing with Dartz, her coming from another world to do so.
In Yugioh GX she shows up on a night where cards can become real and plays against one of the characters (Cyrus) in a duel. She uses a spellcaster deck which focuses on her own card which is quite meta.
In the Yugioh GX Tag Force games she can be battled during the festival and appearing in a forest behind a waterfall. You can recruit her as a partner if you give her a Golden Eggwhich :D