>>281761213
It's because it's a gradual process. He acts more and more differently the longer his life goes on. At the end of the series, he goes to the void world (Mangod's realm) one last time, and his appearance (his soul) is completely changed. At some point, presumably after he lived as Rudeus Greyrat for longer than he had his previous life, he stopped being the previous life and started being Rudeus Greyrat. If you'd actually read the novels, you'd know this. He changes into a completely different person that the one he was at the start, going from not not even going to the funeral of his parents who supported him for ~20 years to begging for someone to kill him and leave his family alone.

But you're too focused on the fact that he was what he was at the start of the story to know or even comprehend that.