I like this more than any of the other gestures of affection that happen between them - the dramatic and sporadic reunion hugs and whatnot - because it’s a very natural, unselfconscious gesture. Mokuba stays up and sticks around to keep his big brother company while he strategizes over Duel Monsters. When he falls asleep, Seto blankets him with his coat.
(Maybe Mokuba tried to engage him in conversation about the day’s events - what did you think of the other duels, are you ready for tomorrow - but Seto’s so absorbed in the God Card that he gives brief, distant answers, and Mokuba settles for just watching him fiddle with scenarios and combos. A silence falls, long and cool. The ship lights go off. The only sounds are the faint hum of the engines and the subtle clicks of keys. Seto glances over to see Mokuba slumped over, fast asleep, breathing softly. He gets out of his chair, shrugs off his coat, and drapes it on Mokuba with studious care, trying not to wake him up. And then he gets back to work: there’s a tournament to win.)
LIKE…. even when Seto stays up late to obsess over God Cards he’s still taking care of his little brother; even when his big brother is wrapped up in strategies and card games Mokuba is still there for Seto. Also, it’s a scene so removed from the usual plot drama (brainwashing!! kidnapping!! forced flashbacks to their traumatic childhoods!!!) that
by the power of headcanon I kind of take it as a nice indication of how they normally interact when they’re alone together. Small, unspoken gestures of affection. Who said they were allowed to be this cute.