This is one of the trickier phrases in Japanese that is very commonly used in different situations and hard to translate, but in this case I am fairly sure it just expresses that you will respect your partner and play fairly
Look on the left side of the card, directly underneath the art
UR Ultra Rare Dark Magician Girl X DARK Level 6 [ SpellcasterEffect ] ATK 2000 DEF 1700 Gains 300 ATK for every "Dark Magician" or "Magician of Black Chaos" in the GYs
Whenever an opponent casts a spell with mana value, power, or toughness equal to the chosen number, that player loses 2 life and you draw a card
4 Silverquill, The Disputant A Slight Mismatch Silverquill, the Disputant, is up next, and it's largely a result of the card feeling as though it doesn't quite fit the archetype, even if it's still a decent card in play
While both the artwork and the cards strength have not stood the test of time, Mewtwo ex remains a relatively rare and nostalgic card, commanding a decent price on TCGplayer