"Any card—so long as it exists—has a power that is needed."

eliaspsuedo:

Looks like Yuto and Yuya got her Polymerization card right?

Actually, look carefully she has two Polymerization cards.

(It’s behind her Fusion monster)

Yuya picked up the one Yuto dropped when he vanished, that’s why it was in the corner when he found it. So both Yuya and Yuzu have at least one copy (as usual for Yu-Gi-Oh!) of Polymerization right now

Besides, both of them have access to a supplier for more!