I don't think so! I attack your Kirby gif with Dory! Who is equipped with an Impressive Wing Span! Yeah, I need a duel, too, and I don't think I've faced you in a while. You wanna duel?
Not bad. I summon Stitch gif and Hercules gif!
F*ck the police.
Really? "Your mom" jokes? Oh, Retz. How low you've sunk.
Yeah, I stole your gif. Whatcha gonna do about it?
You have Thunder-End Dragon (materials are 2 Blue-Eyes), King Dragun, a Blue-Eyes, and 2 facedowns.
Fix'd that for ya. And, yes, he gets to summon his monster.
Alright, we've got an answer from Jaden. The owner gets to use the effect.
Actually, I was wrong there. "You" doesn't always refer to the owner. I'll message Jaden and try to get it settled.
OK, when a card says "you" it refers to the owner, so, since it wasn't sent by the owner, it's effect activates.
When a card is sent to the graveyard, it is sent to the owner's graveyard. In this case, the owner is Axel. So, when Kaiba sent Leyvaten to the graveyard, it was sent to Axel's graveyard. Leyvaten's effect activates in the graveyard, so Axel would be the one to use the effect.
