A/N: This doesn't quite agree with anime-canon, as Kazuma gives Yuma the key after Yuma is crying over losing a duel. I'd love to say its manga-canon…except I haven't read the manga. :D So this is just a fun little alternative "Yuma getting his key" for some Tsukumo family time, using the prompt "unexpected gifts" from the Yugioh ZEXAL Fanfiction Challenges Forum.
Key and Box
It was normally Akari to whom they gave their newly discovered treasures too, so Yuma hadn't expected his father to hand the gold-coloured key to him instead of some brightly coloured toy as was the norm.
Apparently, Akari hasn't either because she scowled at his expression. It was her interest, after all, to explore the origins and tales that circulates those artefacts – from mysteries her parents chased all over the globe to ones made by her own imagination. But their mother was handing something different to her: a box, with symbols running down its sides and a tiny keyhole.
Yuma looked at the key. It had an odd shape; he didn't see how it could possibly open anything – and certainly not the box his sister was fiddling with. 'What's it open?' he asked his father.
His father smiled mysteriously; it was slightly forced, but only his wife and mother noticed. 'Something special,' he said. 'Something that will make you stronger.'
Akari caught the words and tried to peek, but Yuma glared at her and clenched the key tightly in his hand. 'It's mine,' he said, sticking his tongue out. 'So you're not getting it.'
'Hmmph,' Akari scoffed, turning away and returning to admire her box. 'I'll bet you'll forget about it by next week.'
But Yuma didn't forget – because that was the last thing he ever received from his parents. They disappeared that week, and he didn't see them again for a long long time.