"You wouldn't fool around," Leonardo replied. No matter how much he hates Arion right now, he knew that the number one thing that Arion really care for is family.
Risking the life of his family is the least thing Arion would like to do. "So, do you believe me now?" Arion can't have Leonardo kept on doubting and gets angry at him at the same time. He needs Leonardo to make his plan work. His long-time plan should be on the move now that someone has shown itself as a threat.
"I believe that you care for your family, but what you said earlier is something impossible for me to believe." When he first heard what Arion has been doing all these years, Leonardo finds it hard to believe.
"Come on Leo! I am being serious here! Can't we just get on the same page for once?!" From the moment he met Leonardo, they have never been on the same page. The only person Leonardo listened to is Mary and Crona; Leonardo won't listen to anyone other than them.