Well, cards were always one of the options, but at the time I made that, I only had a vague idea of, "I want her to have one of those weapons that shouldn't be a weapon." At various times, that included cards, a book, an umbrella, and a boomerang. When I made that, I didn't want to pin her down with anything in case I changed my mind.
I'm pretty sure it was Baten Kaitos that ultimately made me decide on a deck of cards.
I set my ATM card's number to "0001" because I'm number one!