I'm not sure specifically as I haven't read it. But it might be about the final adventures of Captain Underpants, perhaps his last heroic deeds, or how he finally retires from his wacky superhero life.
One popular Harlock fanfiction could be the ones that explore his adventures in uncharted galaxies. For example, there are stories where he encounters new alien species and has to use his wits and the Arcadia's capabilities to overcome challenges.
Your local library might also have a copy of the 'capitan underpants final novel'. Libraries are a great resource for finding books, and they may already have it in their collection. If not, they might be able to order it for you.
Aside from Yama and Harlock, there could be other characters like Kei Yuki. She might be involved in some fanfics, especially those that explore the more complex relationships within the crew. Also, the villains or antagonists in the Captain Harlock universe can be important characters in fanfics. They create the conflict that Yama and the rest of the crew have to overcome.
Typically, in 'harlock x logan yaoi fanfiction', there is often a focus on the relationship between Harlock and Logan. It may explore their emotional connection, from initial attraction to deeper feelings. There could be elements of adventure or drama added to the mix, depending on the writer's creativity.
One interesting Captain Harlock Yama fanfic could be about Yama's origin story. Maybe it delves into his past before joining Harlock's crew, his family, and what led him to a life of space adventure. Another could be a story where Yama has to face a personal dilemma while on a crucial mission for Harlock, like choosing between loyalty to the crew and his own moral beliefs.
One main feature is its unique character design. Harlock has a very distinct look with his flamboyant yet menacing appearance. His ship is also iconic, often depicted as a large, powerful vessel. Another feature is the space adventure elements. There are battles in space, exploration of different planets and encounters with various alien species.
It has a space - pirate theme. So, there are lots of space battles and adventures. The main character, Harlock, is probably a very cool and brave figure.
I think it could be Emeraldas. She's a strong female character who often crosses paths with Harlock. Her beauty combined with her combat skills and her own sense of justice makes her really memorable. She adds a different dimension to the story compared to Harlock.