Sadie, known in Japan as Seiko, is a young lady who works in Ms. Dorothy's card shop. Her personality is generally cheerful and unassuming. Her name is never given in the English version of the anime, but she is named in the video games. The Japanese credits of episode 4 do name her. She claims to have little knowledge of how to Duel, but can be faced as a opponent in the GX Tag Force series, in which she is usually a tough opponent.
Sadie works at Duel Academy's card shop alongside Ms. Dorothy. She is pulled into alternate dimension with the rest of Duel Academy. She and Dorothy take responsibility for rationing the food during the ordeal.
In Yu-Gi-Oh! GX Tag Force the player must beat the game with at least five different Tier 1 characters, before they can duel Sadie. Playing through Story Mode in Tag Force, Tag Force 2 and Tag Force 3 with Sadie as your partner unlocks the Booster Pack containing the majority of the most powerful burn cards, matching her playing style.