On the Death Knight side, if they meet Naruto, a character full of life and hope, it could trigger a change in them. They might start to remember their own humanity. Maybe a Death Knight, who has been consumed by darkness for a long time, could see Naruto's unwavering spirit and start to yearn for redemption. This would be a great way to show character development in the fanfiction.
In a Naruto Death Knight fanfiction, the clash of cultures between the ninja world and the world of the Death Knight can also lead to character development. Naruto might introduce the Death Knights to the concept of teamwork and friendship in the way that ninjas have. This could make the Death Knights, who are often solitary and brooding, to open up and form bonds. At the same time, Naruto could learn from the Death Knights' stoicism and their acceptance of a darker side of existence, which would add more depth to his character.
Well, for Naruto, becoming a Death Knight or interacting with them could make him question his own beliefs about life and death. His experience with the Death Knight's view on the afterlife might change how he views his own ninja way. For instance, he might start to think more about the consequences of killing in battle.