He had always thought that Yusei was the weaker one of them two. He had known him for a long time, but there were some things he never did figure out about him, so he simply assumed. Like his quietness. Yusei wasn't one to start an argument, to talk much unless he was directly addressed. To talk much at all.
Jack on the other hand was a very, very argumentative person. Even known to be a bit on the aggressive side.
Yusei was very loyal. As much as he'd like to leave the Satellite behind, the way practically every person would, so that he could gain a better future in Neo Domino, he hadn't left. Because he didn't want to abandon his friends. But Jack was very prepared to, in fact, he had even gone so far to endanger the life of Rally and betray Yusei, in order to gain this better future.
And because of these differences between them, Jack regarded Yusei as the weaker link.
But now, watching him duel against Godwin, the only one out of him, Jack and Crow still standing strong enough to save their worlds , Jack realised that he was wrong.
Out of the two of them , it was actually Yusei who had the most strength .
