One key theme is personal growth. Eris, for example, goes through significant development from a wild and unruly girl to a more mature and responsible individual. Another theme is friendship, as seen in her relationship with the protagonist which endures many challenges. There's also the theme of self - discovery, as she figures out her place in the world and her own capabilities.
The main character is Eris, a girl full of energy and determination. Then there is the male protagonist who has a deep connection with Eris. There are also characters in the places she visits, such as the lords and knights of different regions. These characters interact with Eris and contribute to the overall story of the Mushoku Tensei Eris light novel.
The length of Mushoku Tensei manga varies, but it has a significant number of chapters. It's hard to give an exact count as it keeps evolving with new releases.