Kaiba usually isn't in the mood to discuss this...but he's feeling rather loose-lipped right now, and so he answers:

"I hope you can teach him someday. It'd be nice to meet him. If Mokuba can forgive me...maybe he can meet him too."

That might be confusing, so he elaborates: "Mokuba would be about sixteen now. Right before I was taken, I...left to chase someone else I'd lost. He was angry with me, afraid I wouldn't come back. He was right, but I never meant to leave for this long. And because my comatose body was somewhere unreachable, he couldn't even find that."

He frowns deeply. That still hurts. "But he's a really smart guy. Determined. Friendly. He's going to do great things...I just hope I can fix this."