Well, the main themes include heroism and sacrifice. The characters in the Fire Force light novel often have to put themselves in danger to protect others from the Infernals. And there's the exploration of a post - apocalyptic world where fire has a significant and often destructive role. It also delves into the moral and ethical aspects of using power, especially when it comes to the special abilities of the Fire Force members.
Shinra is definitely one of the most popular. His unique power of the 'Devil's Footprints' and his determination to find answers about his family and the mystery behind the Infernals make him a great focus. Also, Arthur with his delusions of being a knight but also having real strength is a fan favorite. His comical yet brave nature often gets explored in fanfiction. Another popular one is Maki, especially for stories that focus on female empowerment within the Fire Force universe. Her strong will and powerful combat abilities make her an interesting character to write about.
Shinra Kusakabe is quite interesting. His ability to use his feet as jets gives him a unique fighting style. Then there's Arthur Boyle, with his delusions of being a knight, which adds a touch of humor to the story while also making him a formidable fighter. Also, Captain Obi is interesting as he leads the 8th Company and has to deal with all the eccentric members under his command.