Yuga slipped one of Mimi’s cards into his Deck.

RD/SBD1-JP032 なぞの手錠 Nazo no Tejyou (Mystery Handcuffs)
Normal Trap Card
[Requirement] You can activate when your opponent Normal Summons a monster.
[Effect] Choose 1 face-up monster on your opponent’s field. During this turn, while the chosen monster is face-up, your opponent cannot Tribute the chosen monster for a Tribute Summon.

RD/SDB6-JP024 Mystery Handcuffs

RD/SDB7-JP032 Mystery Handcuffs

Source