Oh. Right. Yuugi. Of course he's worrying about Yuugi, they'd ended up getting along after everything, and Seto would be lying if he admitted that didn't bother him. He's not the kind of guy that likes to share his brother, evidently. Or maybe he's still suspicious of Yuugi in general.

It could be both...couldn't it?

(he's jealous)

"If you're so worried, then I can send Isono to check on him," Seto offered blandly. "People are allowed to call out sick. Maybe it isn't so bad."

He's attempting to comfort his brother, and it's very awkward, because he's not as sympathetic as Mokuba. Thanks Gozaburo, you did a great job raising a monster.