Catherine is the ex-lover and former stage assistant of Arkana.
Catherine was the assistant to the famous stage magician, Arkana, and the two fell in love with each other, having built a steady living together. In the English anime, Arkana claimed that the two were engaged.
Arkana became injured after a dangerous trick went wrong, leaving his face and reputation permanently scarred. He became too stressed with his own dilemma that he came to ignore Catherine's love at the time, and in a selfish move, he drove her away in anguish.
After she left, Arkana came to realize his mistake too late, and joined the Rare Hunters, where Marik Ishtar promised to reunite him with Catherine via the power of his Millennium Rod, should he succeed in beating Yugi Muto in a Duel. He got Arkana to duel Yugi and pretended to have Catherine waiting for him behind a curtain after he wins. It was clear that losing Catherine had a negative impact on Arkana's psyche, as he acted thoroughly sadistic with Yugi and his own monsters, and was willing to do anything to win in order to regain Catherine's love.
However after the Duel, in which Arkana lost, he rushed up to the curtain only to find a doll. By that time, Arkana lost what little sanity he had left, and caressed the doll as though it was the real Catherine. (In the English dub, Arkana lamented that he has been deceived, then deluded himself into thinking that Marik has the real Catherine somewhere else.)
In the manga, Pandora was said by Marik to have lost his lover after the awry magic show, but did not elaborate on the situation, and Catherine was not introduced into the story-line. As such, she is an anime-only character.