Maybe it's because I haven't gotten that far in the manga yet but I really wished we got to see a progression of subtle individualism and quirks as Yami progressed to the Pharaoh to finally Atem.
Atem being kind of actually really petulant when he doesn’t get his way. Atem suddenly using his Authortive Finality Pharaoh voice with Yugi because that worked in getting people to just listen and to not argue with him any further (instead it just leads to a very unimpressed, indignant Yugi almost sending Atem to the metaphorical couch for using such a tone with him).
Atem beginning to take a more active part in choosing outfits and accessories when Yugi leaves him to dress them for a day. Less restrictive clothes—baggy jeans and shorts, loose tank tops and t-shirts. If he can get away with sandals he will. Yugi ends up with a lot of cropped shirts. Lots more gold jewelry, with interesting designs and (gaudy) costume jewels and stones, because there's no way Yugi can afford actual gold pieces. Atem's favorite colors being these extremely pigmented blues, purples, and reds. It becomes obvious when Atem has dressed them for the day versus when Yugi has.
Similarly, Yugi's makeup depends on who was in charge. It's not kohl but Atem prefers the pencil eyeliner over wet. He also likes adding color, not just a smudge of black liner like Yugi does. Strokes of vibrant blues and yellows, reds and pearl white, around the corners of the eyes and across the lid. It often brings out the blue-violet variations in Yugi's eyes. Yugi thinks it's too much, but Atem finds it very flattering.
Atem occasionally cracking jokes that make absolutely no sense to everyone else and only to Atem because it's missing the context of time period (you'd have to be there to get it) but also the translation from old Egyptian Coptic is not direct.
Atem isn't easy to make laugh, having a habit of hiding his amusement behind a closed fist or turning away. He's just a sincerely serious person, stoic by nature and how he had been groomed to fit the expectations of a future ruler. It's important to him that he appears almost unreadable, a little mysterious. Atem does not feeling vulnerable, and he has a habit of being automatically suspicious of anyone new he meets until they prove themselves worthy of his time, attention, and friendship. Someone is always wanting something from you until they prove otherwise, kind of attitude. Yugi doesn't mind the caution, but he does find Atem's way of thinking just a bit overly critical and cynical.
Yugi being the only one who can open Atem up like a book and leaf through his pages, who knows when something is bothering Aten. Yugi being the only one to make Atem laugh genuinely, sincerely, openly. It's deep and contagious, one of Yugi's favorite sounds.
Atem slipping into ancient Egyptian Coptic, both subconsciously but also because this little smart ass knows no one else can understand him. He also slips into it when he forgets a word, or when he's feeling the exhaustion in the shared vessel of Yugi's body that he'll smash Japanese words with Coptic.
Atem trying to teach Yugi Coptic. Of course Yugi wants to learn the naughty words first (of course Atem relents).
Atem having Yugi save up enough to buy certain oils and incense. Yugi starts smelling faintly of incense, smoke, and a gentle, earthy musk from the oils.
Yugi can't do sports for shit or prolonged physical activity but when Atem is in charge? He suddenly becomes a real favorite to be picked between teams. Atem loves being physical, actually enjoys the strain and the burn of overusing muscle, the cool sensation of sweat. It may not be his body, but he remembers what it was like to have his own body. He was strong, he was fast, agile.
Atem hardly moved by any talk of dirty magazines or risqué videos that Yugi and Jonouchi share. During his time, there was nothing shameful of the naked body, it wasn't taboo, it just was. He also recalls his fathers harem, and what all that would have entailed for him had he been allowed to rule longer as Pharaoh, had been allowed to grow older and into his own as a young man. Nakedness and sex is hardly something Atem hardly blinks at.
Atem having an appetite, memories that make him feel hunger and his mouth to water. Of course he has more of an affinity towards middle eastern delicacies than the simple and greasy burger Yugi favorites.
Atem having a temper, only made much more real by retaining his name and past. Yugi having to work with Atem on quelling it, having to often be the balm and water to Atem's white hot temper and anger. It still rears it's ugly head if Atem believes an injustice has happened, especially when it involves Yugi. However his anger is not always righteous, his punishment not always fair. I'm talking season 0 mind crush wicked games temperamental Atem. This often leads Yugi being the one to try and ground Atem, plead and demand for mercy and forgiveness.
Atem's obvious love for animals, for cats and dogs, and especially for horses. With Yugi, he recollects all his memories of learning how to ride, of chariot rides and hunting parties. Of learning how to shoot a bow or toss a spear while moving.
Yugi having to share in the nightmares and traumatic memories of war, of watching the innocent suffer, of bloodshed and people Atem loved dearly getting hurt or dying. Yugi snapping awake in a panic of sweat and tears, and Atem makes them feel as if they're going to be nauseous. The memory of dying, of being locked away in darkness. Yugi cries for Atem, grieves for him, and in the end they just try to comfort each other.
Atem being much more studious than Yugi, naturally just more inclined to focus and take certain responsibilities and obligations seriously. If I had to go to all my studies and learn boring geopolitics and border relations, you can master basic algebra partner. Atem no longer being just a passive study buddy and curious observer over Yugi's shoulder. He encourages and presses Yugi to do his homework, will stay up all night with him for tests and quizzes.
Atem being stubborn. Atem being socially awkward in a casual setting because almost all his memories are of when he was expected to behave like a prince, or the future Pharaoh, or to be a Pharaoh. He's just a little out of his depth. He's almost shy but he can also be a little too blunt and a little too direct. He can take things for face value, and doesn't always understand a modern punchline.
Atem is jealous, and possessive, memories of not being expected to share what belonged to him and him alone. Pharaohs were not left to want, to share if they did not want to. Yugi is his, isn’t he? Yugi is not just his vessel, but his friend, more than just his friend, his equal, his heart. A treasure far removed from the golds and silks and fertile land of Kemet. So he gets into these moods, whenever he feels as if his relationship and bond with Yugi is threatened. When he remembers he doesn't get to keep Yugi forever, that one day they'll separate and Atem must return to the afterlife, and Yugi stays behind, to start his own life, to be loved by someone else. It's such a powerfully ugly feeling, sometimes it even unsettles Atem.
Atem dealing with the grief from memories of losing his father. Being taken away from his life and friends. And the inevitable separation from his vessel, his partner, his Yugi. His heart. He prefers to deal with these feelings alone, however Yugi is mighty determined and persuasive in making sure Atem doesn't get too stuck in his brooding, in him isolating himself.
Atem preferring hot days to cold, however he misses the dry, arid climate of home.
Atem loves to sunbathe, however that proves difficult with how Yugi's skin is so susceptible to sunburn.
Atem recalling prayers to the gods, to Ra, Amun, Osiris, and Horus. Yugi subconsciously reciting a chant in his head like that of a random melody to hum, not understanding it and not initially knowing where it came from.
Through Atem, Yugi can understand hieroglyphs, can read them and make sense. It's magical to Yugi, but it's just like reading a bulletin board to Atem.
Atem does have a bit of an ego befitting of a king, which can be easily stroked like a spoiled cat or bruised and battered by just the wrong look. Yugi likes to tease him about it.
Atem is also a massive tease, at least with Yugi. He loves to make his partner fluster and turn red, actually enjoys how Yugi doesn't ever treat or address him like the Pharaoh he was is, but just like Atem could be anyone else who is intimately close to Yugi. Compared to the memories, the way Yugi behaves with him can be refreshing, to a point. But only Yugi, everyone else can get bent.
Atem becoming more human. More flawed. Defined. Atem will always be Yugi's other me and it will always be them bleeding into each other and their souls are too far entangled but he is also Atem, and isn't that just as amazing and terrifying? It means something. Being Atem means something.
