What do you think Atem's relationship with Yugi's grandpa would be like after Kaiba brings him back? I don't really remember watching them interact directly in the anime, but I do remember that Atem would switch between referring to him as Yugi's grandpa and just plain calling him Grandpa, and Grandpa Mutou seemed to recall meeting Atem's... consciousness? spirit? whatever when he was stuck in his tomb.
Atem’s not quite sure how he fits in when he comes back.
He’s not a fan of being on his own in the Kaiba brother’s empty house all day, its silence unfamiliar and unwelcome. Instead he often spends his days at the game shop with Yugi.
On the rare occasions he sees her, Yugi’s mother seems to be not-so-fond of him - he doesn’t know if it’s that she knows he has put her son in danger so many times, or that she simply doesn’t like that a poor copy of her boy is hanging around. Maybe it’s the hand-holding.He feels unsure with Solomon too, even though he’s never been anything but pleasant. He asks him for advice on which games he should buy in, gets the two of them to test run new ones, even lets him handle the shop on his own once when Yugi’s running late. Solomon never asks him any questions, but occasionally he finds himself talking to him about things. Things he feels like he shouldn’t remember. Solomon never brings these conversations up, but seems to recall every detail when Atem next gets gets too talkative over a game of Egizia.
Atem often catches himself about to call him Grandpa, before quickly shifting to ‘Solomon’, or sliding a ‘Yugi’s’ in front of it. He’s spent so long with Yugi it just feels natural. Once he slips and he calls him Grandpa right in front of him, and he stammers his correction while he feels himself go as red as the tips of his hair. So it means all the more, when a delivery man comes in later and he unnecessarily introduces them as his “Grandsons, Yugi and Atem.”
He doesn’t come out of the store cupboard for a while.