Yuusei had no proof that wasn't Kiryuu's goal. No proof, but a belief? Yes. Maybe it was because of what he had told him told after their second Duel of Darkness. Maybe it was because it just-- didn't add up. If he was to kill the Signer, he could've said so outwardly in the message; it wasn't like he would've run away. And on top of that, he just wanted to believe that the old Kiryuu, his friend and leader, was still there.

"... I trust you," came Yuusei's short answer, "You're my friend."