I do have a few headcanons about Marik.
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When Marik was a child, living underground, he rarely (if ever) wore shoes, so he grew accustomed to moving around barefoot. Because of this, he still prefers to go around barefoot whenever he can, and it took him a long time to get used to the feeling of wearing shoes.
When he left the Tombs with Rishid, he continued to go around without shoes, since he rarely interacted with people outside of his own Gouls. While prowling through cities at night, however, he learned the value of wearing shoes after stepping on shards of a broken bottle, needing several stitches (which Rishid provided assistance with). Afterwards, he worked harder to try and get used to the feeling of wearing shoes more often.
I think I have mentioned this one before, but I also believe that Marik is none too fond of elevators (along with his siblings, to varying degrees). He will use one if he needs to, but he much prefers to simply use stairs. Despite adjusting to living in cities and traveling around, he still gets a feeling of unease in elevators. Planes also trouble him in a similar way (being confined in an enclosed, metal-walled space), but he can sleep through flights if he needs to.
He will use elevators when he needs to; they don’t make him panic or leave him unable to function as he normally would, but he still prefers stairs if they are an available option.
Marik’s interest in ice cubes (or ice in general) does not branch into a sexual fetish in my headcanons. Rather they intrigue him, since the concept of ice was entirely foreign to him as a child. He enjoys the chill they create in drinks, as well as the texture they have when touched, and how an ice cube melts when held.
The first time he encountered ice cubes after leaving with Rishid, he was more than a little startled. He did not understand what it was, or why it was in his drink, and ended up spending most of that meal time holding an ice cube or pushing another around the table top as it melted. He got used to it over time, but it is still something he takes a secret kind of delight in toying with.
I suppose my last consistent headcanon would be that Marik frequently woke up with terrible headaches. These headaches were caused by his darker side, in a way, so they began when he was a child, and only stopped after the Battle City events. His darker side had no way to directly influence him while it was dormant, but it was eternally existing and stirring just beneath the surface of his mind, always watchful for the smallest opening, the slightest weakness, to give it the chance to ‘come out’ again. This alone would often give him headaches, particularly after a fitful night of sleep.
When Marik lived with Rishid after they left the Tombs, Rishid became accustomed to bringing Marik a glass of water and some pain medication every morning. It was not always needed, since Marik did not always get his headaches every morning, but it happened frequently enough that it became a standard practice for Rishid.
[I hope that answered your question! I have many headcanons about characters, but a lot of them vary depending on situations or other factors, so these are the main headcanons I have for Marik that stay the same across the board. Thank you for asking!]