Well, one interesting aspect could be his character development. He might start as one type of person and gradually change throughout the story of the light novel. Maybe he has unique relationships with other characters that add depth to the plot.
Sure. There could be his family members. For example, his father or mother might play important roles in his life and in the story. Their influence on Paul can be a key aspect of his character development.
One of the main themes is growth. Eris Greyrat goes through significant personal growth throughout the novels. Another theme is relationships, like her relationship with Rudeus Greyrat.
Well, 'Mushoku Tensei' light novels where Eris Greyrat is a key character are great. They depict her growth from a young, headstrong girl to a more mature individual. There are many exciting arcs involving her, like her training and adventures in different lands.
As we don't have the details of the Paul Rosenau story, Paul Rosenau could be anyone. He might be a local hero in a small town where the story is set, known for his kindness or some great deeds. Or he could be a character with a complex personality, whose actions drive the plot of the story.
Paul could be a descendant of an ancient sea - faring family. There's a prophecy that one day a member of his family will meet Poseidon again. So, he has a special connection to the sea, and when he meets Poseidon, it's like a reunion of old times, even though Paul may not be fully aware of his family's history at first.