The villains are always a big draw. Characters like the Joker are extremely popular. Their chaotic and unpredictable nature makes for great storytelling in the fanfiction. Writers can come up with new schemes and interactions between the villains and Batman in this medium.
Batman himself is of course extremely popular. He is the central figure, and in 1939 fanfiction, people love to explore different aspects of his character, like his detective skills and his relationships.
Robin, in all his incarnations, is also a popular character in fanfiction related to Batman. He represents the sidekick aspect and the growth and development that comes with it. In Batman Beyond, Commissioner Barbara Gordon also features prominently in some fanfictions. Her connection to the original Batman era and her role in the new one can be explored in interesting ways.
Gwen Stacy also features prominently in many Spider Man Unlimited fanfictions. Her relationship with Spider Man, whether it's a romantic one or a platonic friendship, is often explored in different and interesting ways. Writers like to play with the dynamics between them, adding depth to the story.
Catwoman often features prominently. Her complex relationship with Batman, both as an adversary and sometimes an ally, makes her a fan favorite in fanfiction. The tension between them adds a lot of drama to the stories.
Batman and Robin themselves are of course very popular. Batman's brooding nature and Robin's youthful energy make for an interesting contrast that many fans like to explore in fanfiction.
The Joker is also quite popular. When Batman's identity is revealed, the Joker's reaction can be explored in many different and interesting ways. He could either be overjoyed at finally having the upper hand or he could be disappointed because the mystery is gone.
Batman himself is obviously one of the most popular. Fans love to see how he would be written in the Arrowverse context. Another popular character would be Alfred. His relationship with Batman and his role as the caretaker and advisor always adds depth to the story. And also, characters like Catwoman might be popular as she brings in the element of mystery and romance to the fanfictions.
Ben Tennyson is obviously one of the most popular. After all, the whole story revolves around him. Gwen is also quite popular as she has her own powers and a complex relationship with Ben. Kevin Levin is another favorite as he has his own backstory and his character development is interesting. He can be both an ally and an adversary at times.
Obviously, Batman and Robin themselves are very popular. Their dynamic duo relationship is a major draw. Batman with his brooding and intelligent nature, and Robin with his youthful energy and loyalty.
Max is quite popular in 'Batman Beyond Unmasked' fanfiction. She is Terry's friend and often gets involved in his adventures. Her character provides an interesting contrast to Terry's more serious and secretive nature. Another popular character is probably Commissioner Barbara Gordon. She has a connection to the old Batman days and her role in the future Gotham is always an interesting topic in fanfiction.