Ten things that the Kaiba-household does not do.
Mokuba does not really sneak into the kitchen to eat a quiet sandwich by himself. He gets enough food at dinner time, after all, and doesn't need to be fed any more. He doesn't really crave the quiet - he loves his big brother more than anything in the world. So he doesn't go downstairs when all is dim and silent to listen to the clock ticking quietly away, and he doesn't think about things like relationships or his big brother's job or the fact that he's less lonely when Seto is asleep than when Seto is awake and working.
Kaiba does not have trouble sleeping. Nightmares do not keep him awake, and neither does stress of any kind - stress about his brother, about his work, about a worker he is unsure of, about people from school... no, he doesn't stress at all. But when he doesn't feel like sleeping, especially when it's raining, he certainly does not get into his car and drive, clenching the wheel hard so that his knuckles turn white. He doesn't drive around the park, and he doesn't see Joey at the park, and most of all he does not see Joey at the park crying. Because really, Joey wouldn't be at the park at that time of night, crying of all things, and Kaiba wouldn't be there at that time of night either.
When Kaiba goes to school, he does not sit beside Joey because of the seating arrangements. He doesn't even look at Joey through the school day, other than to glare at him if Joey does something stupid like he often does, like trip over and fall on Kaiba by mistake, or laugh too loudly and almost distract Kaiba from work on his laptop (only almost, because Kaiba never allows himself to be distracted by such nonsense). He doesn't notice, then, if Joey is happy or sad or angry or whatever else the Mutt might be feeling on any given day. Kaiba has no time to notice things like that, and he doesn't care.
One day at school, Kaiba is far too busy doing something urgent to see that Joey is dragged off by some boy who is their age but not in school uniform. He doesn't see that the boy looks like a stupid punk, and he doesn't see the punk push Joey around the corner, or that Joey is shoved hard up against the wall. He is turned towards his laptop as usual, so that he certainly doesn't see Joey yelling something, or the boy leaning dangerously towards where Joey's face must be. And above all - because Kaiba is most definitely not watching - he doesn't see the unknown boy lean in to kiss Joey. Kaiba's hands do not clench into fists, his eyes do not narrow, and there is no frown marring his forehead. He certainly isn't watching when Joey shoves the boy roughly away and drags a hand across his mouth, a disgusted look on his face. Kaiba is even concentrating so hard that he doesn't hear Joey tell the boy to fuck off. Because he didn't hear this, a slightly pleased expression does not make its way onto Kaiba's face.
Mokuba doesn't ask when Joey is sitting in Kaiba's bedroom and flinching as Kaiba applies a cold cloth to what might be an injury. Mokuba doesn't poke his head around the door in time to catch a quick exchange of words from his big brother into Joey's ear that make Joey look flustered, angry and pleased all at once. Mokuba doesn't stay there to also watch Joey sway a little, and isn't still watching to see Kaiba put an arm around Joey to steady him, an odd look on his face. He doesn't watch Kaiba keep his arm there longer than necessary, and a peculiar grin doesn't settle on Mokuba's face when he doesn't see Joey not pulling away. Mokuba is an angelic creature and would of course never spy on his big brother.
Kaiba and Joey do not mix very well at all, and so when they are together - doing homework, because the teacher had infuriatingly designated Kaiba to be Joey's personal tutor if Kaiba wanted to pass the year - Joey only made Kaiba angry. Joey did not cause Kaiba to smile, because Kaiba was not happy with Joey being there and his only intent was to finish the one-hour period of tutoring and get Joey out of the house as soon as possible. And since Joey was so annoying by nature, he did not make Kaiba smile, or laugh, or blush, or feel anything other than exasperation. Joey did not smile in such a way that the smile was catching, and he was not so amusing as to make Kaiba laugh in any way other than derision.
The housemaids (for there must be housemaids in a mansion the size that the Kaiba's have), do not tiptoe about and eavesdrop. It is unbecoming to be eavesdropping, and if they did such things then they were quite sure that Master Kaiba would show his extreme displeasure. So when a certain blonde boy with a loud voice and cheerful smile comes in sometimes - perhaps so that Kaiba can tutor him, and perhaps not - they do not clean outside the room the two happen to be in any more slowly than usual, ears straining to perhaps catch the sounds of two boys who don't seem to be getting all that much work done. The housemaids do not giggle about it among themselves, either.
The butler at the Kaiba mansion doesn't tell wild stories about the time Kaiba came flying in from the rain, soaking wet, to lock himself in his bedroom - presumably to do work. The butler never saw anything like that, and he is quite sure that Kaiba, whose mind is never anywhere else other than on his own work or his younger brother, would never look just excited as Mokuba did on his last birthday. He knows that there is no resemblance between the two brothers at all, and that identical expressions would not occur to anyone - not in their wildest dreams. The Master Kaiba was not a child, and he would certainly never look like one, least of all like his younger brother.
Joey does not eventually come to stay in the Kaiba mansion. Kaiba doesn't invite Joey of his own will, and even if he did, Joey would never accept because perhaps the one thing the two have in common is that they are both proud. So Joey doesn't live there, and Kaiba never invites him because he knows things would never work out. Joey would always be an idiot, and Kaiba would always be too busy to have someone so very annoying constantly around the place. And besides all these things, even if Kaiba did like Joey - which of course he doesn't, that's Mokuba's over-active imagination - it just wouldn't work. That would be like thinking that things have happy endings. Kaiba is a cynical bastard, and does not believe in happily ever after's.
Anyone who has known the Kaiba mansion and the people who live there - the elder boy, his younger brother, the housemaids, the butler... - everyone who knows the house and the residents know that if everything else changes, one thing will always remain the same. The most important thing to note about the Kaiba household is that the people there do not always tell the truth.