Note: This has been posted to my AO3 account (dragonsilk over there) for ages. Since it obeys the very strict rules of this site, I figured I'd cross post.
I like to write out my half formed ideas. It helps make them whole. Even if this may not read like a whole thought.
I often write Mokuba as super social and outgoing. It's because I feel like he might try to compensate for his brother's most obvious weakness. Mokuba can take over any social functions and let Kaiba focus on what he does best. Run the company and invent things. Meanwhile, Mokuba can charm the pants of potential investors (figuratively and literally if he desires). That's not a skill that Seto has. Mokuba tells himself this is evidence that he's not a burden.
Sometimes, Mokuba felt like a burden. It wasn't a constant feeling, but it was a persistent one. The sort of feeling that lodged itself in the darkest corners of your head and only emerged on occasion before going back into hiding. Mokuba didn't know when he first thought that Seto's life might have been better without him, but he did know that it dated back to the days of the orphanage.
Anyone who visited the orphanage wanted Seto. Parents were constantly requesting him for adoption. They wanted him to be part of their families... but nobody ever wanted Mokuba. They never had room for a second.
Seto and Mokuba. They were a package deal. Seto said he would never leave Mokuba behind. He promised that that they would stay together, even if that meant that they never got adopted.
Every time Seto promised not to leave him behind, Mokuba would return the promise even though nobody ever asked for him. The action felt empty to Mokuba. Seto's promise was real, while Mokuba's was meaningless. He couldn't imagine leaving Seto behind, but nobody would ever ask that of him. Nobody but Seto wanted him.
Even Gozaburo hadn't wanted Mokuba. Until he needed Mokuba's shares of Kaiba Corp, Gozaburo never even glanced twice at Mokuba. He was just the kid that came along with Seto. Part of the package.
Mokuba watched as Seto worked himself to sleep, night after night, constantly trying to keep up with all the lessons that Gozaburo gave him. Mokuba watched as Seto stopped smiling. He couldn't do anything but wonder if Seto would have been better off without him. If he didn't exist, or maybe if he died with their parents, would Seto be happier? Without Mokuba to hold him back, Seto would have been adopted by one of the many nice families who'd wanted him. Mokuba knew Seto could have had a normal childhood without him. Seto could have played with toys and games. He could have watched TV. He might have had friends.
Instead, Seto worked, and worked, and worked. He rarely slept. He stopped smiling.
It didn't get better with Gozaburo's death, either. Mokuba thought that maybe Seto could relax, but Seto couldn't stop working. He couldn't pause for a second despite his freedom from Gozaburo. He had the entire Kaiba empire weighing down on his shoulders, and he was determined to break it apart so he could rebuild it from scratch.
Mokuba watched his brother give up his childhood and step into the shoes of an adult. He worried about whether or not he was worth it. Maybe Seto should have let one of those nice families adopt him...
Mokuba watched as his brother began to rebuild Kaiba Corporation into a gaming empire. He admired his brother's determination to turn Kaiba Corp. into his own company. He let himself hope. Maybe if Seto were able to turn it into a successful gaming company, then maybe he would be able to play more games. Maybe then he would be able to enjoy himself again.
Seto started playing duel monsters, and Mokuba stopped feeling like a burden. Mokuba relaxed. His brother could finally enjoy life.
It took a while, but Mokuba slowly realized that Seto didn't actually enjoy playing duel monsters. With growing horror, Mokuba watched as Seto took down players methodically, one by one, until he was standing at the top.
It was brilliant. As Seto gained more renown for his dueling skills, the company became more successful.
Mokuba wondered if he would ever see Seto smile again. He wondered if Seto would have had a happier life without him.