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It wasn't like Jounouchi was a thief, he just liked having little tokens of Kaiba to remember. It started with the Kaiba Corp pens, those amazing things that wrote anywhere and on anything like some kind of space age super pen. He'd take a couple here and there from Kaiba's never-ending collection of them.
Then he went a little bolder. He started taking cards from the new packs Kaiba brought home. Never any rare ones; Kaiba would've noticed that in a heartbeat, but the duplicates, the ones that Kaiba would never dream of including in his deck. Jounouchi had a deckful of cards that he'd taken from Kaiba. He sifted through them often, remembering which pack each came from, what Kaiba had said about them.
He started getting even bolder. He'd take things while Kaiba was there. When he'd look away, Jounouchi would quickly take something off his desk, his plate, his table. Once Jounouchi had stolen his phone when Kaiba was up for a moment. Kaiba spent five minutes looking for it before Jounouchi returned it.
It was all fun and games, until Jounouchi's things started disappearing.