As we know yami Marik was an evil part of Marik that was born from being forced into a life he didn’t ask for

And seeing his strong bond with his stepbrother got me thinking that Marik is possibly autistic meaning he has a mental disability I know because I have adhd and a bit of autism and in some movies and tv shows it very will hinted that split personality can be developed from forms of abuse as a means to help people coup with their environment which makes since because look at Marik and ishizu on how they were raised by their abusive fanatical dad that is definitely a common phenomenon to cause a split personality

Which could’ve been avoided had Marik father given his kid choices in their lives or have his son tested for Autism which he wouldn’t du to the tomb keepers belief that they must avoid the outside world at all costs proving he was willing to ignore all that in his obsession to ensure the pharaoh legacy