There is only one chair in Seto Kaiba's office.
There is no room for anyone else to sit in the space that he has claimed: it is the CEO's way of ensuring that he has no equal in this room. Mokuba understands, then, what it will mean for him if he stays past the point where his legs can stand—but he does so anyway, telling himself it is important that he learn to understand the affairs of his brother's business.
Niisama inevitably sneers, whenever he hears Mokuba make any mention of his own significance to the company. Mokuba tolerates the insults because he understands his brother cares for him, no matter what he might say or refuse to acknowledge in his silence. Niisama's cutting words are meant to make his younger brother stronger, to strip away the softness that makes Mokuba regrettably weak. Mokuba will be strong for his Niisama. He will remain at his brother's office while he works, taking the older Kaiba's continued tolerance of his presence as an indicator of some lingering concern. A concern that Niisama never shows anymore, through any of his actions or words.
Mokuba's feet inevitably grow weary beneath him as the hours pass. He does his best to ignore them, as he attempts every other night—refusing to slouch as the sun fades into darkness, to whine about the lateness of the hour. Niisama does not tolerate weakness; not from Mokuba, not from anybody else. He especially does not tolerate weakness from himself, which is one reason why Mokuba admires him so and tries to emulate him.
Niisama is perfect, Mokuba tells himself adamantly each day. Niisama is unrivaled and second to none, Niisama has no equal. To match Niisama is to accomplish the unthinkable. And when Mokuba's weaker legs invariably strain past the point of no return, well...
It's only natural, then, that his small frame should eventually succumb, once his trembling muscles have been exhausted to the brink of collapse. Mokuba hates being bowed and weak, unlike his brother, but it does not delay the point where his body simply sinks beneath him of its own accord and surrenders its position, standing proudly by his brother's chair. Mokuba's legs give way at last, knees sinking reluctantly to the plush office carpeting with relief. And he accepts it.
A man like Seto Kaiba does not tolerate anyone, let alone a mere child, to sit as an equal in his presence. Exhaustion and childish weakness will not change that. But he lets Mokuba remain in the room with him regardless, and Mokuba construes this as acceptance, drinks in the perceived affection as greedily as the sweets he craves. It might be degrading, sitting like this on his knees at his brother's feet, but Mokuba does not resent it. On rare occasions, he even feels Niisama's touch on his hair, a hand reaching down to grip absentmindedly in the raven locks, and Mokuba feels a thrill of affection at the contact—despite the fact that it is rough and painful, yanking in the fisted hair with the gentleness of a brute.
After so many desperate years with Seto as his only anchor, his only kindness, Mokuba has become so blinded by devotion that he is incapable of understanding the depths to which his older brother has dehumanized him. If Mokuba is a pawn, a pet or toy kept solely for his Niisama's sick amusement, he will hold onto his obsessive faith and not complain. He'll do the best he possibly can for his Niisama, learn and imitate and worship blindly, following always at his brother's heels like a beaten dog as Seto walks his coming path to hell. Mokuba will take the punishment and wait, expectantly, for his loyalty to finally be rewarded.
And one day, soon—when that terrible path of his elder brother's shatters, and rescinds—his devastating allegiance will be returned.