I don't know which novel you're referring to because you didn't provide any context or keywords. If you can provide more information or clarify what you want to know, I will try my best to answer your questions.
An interesting Blue Spirit Avatar fanfic often has a well - crafted plot. It could be a story that takes place during an unexplored time in the Avatar universe, like what the Blue Spirit was doing during a certain event in the show. Also, good character development for not just the Blue Spirit but also the other characters he interacts with, such as Aang or Katara, can make the fanfic engaging.
A great 'Avatar Blue Spirit' fanfic often has a deep exploration of the character. For example, it might dig into his motives, like why he helps Aang at times. Also, a good fanfic in this regard would have a well - developed plot. It could involve the Blue Spirit facing new challenges or enemies that test his skills and values.
One great 'Avatar Blue Spirit' fanfic is 'The Blue Spirit's Journey'. It delves deeper into the backstory of the Blue Spirit, exploring his training and the events that led him to become the masked hero. Another one is 'Blue Spirit and the Avatar's Bond' which focuses on the relationship between the Blue Spirit and Aang, showing how they come to trust and rely on each other in unexpected ways.
As mentioned before, lemon fanfictions usually contain adult and inappropriate content, so I can't recommend any. It's better to focus on positive and appropriate fan - created works about Blue Spirit and Katara.
I'm not sure specifically as 'lemon' in fanfiction often implies mature or inappropriate content. But generally, it might be a fan - created story involving Katara and the Blue Spirit from the Avatar universe, perhaps exploring their relationship in a more romantic or adventure - filled way without the improper elements.