Well, Issei for sure. His harem - building adventures are a big part of what makes him popular in fanfiction. Rias, with her noble background and her connection to Issei, is always a favorite. And then there's Vali. His cool and powerful demeanor makes him a character that many fanfic writers like to include. He can be portrayed as an ally or a rival, adding a lot of drama to the stories.
Often, the vampire leader is a popular character. He's usually depicted as strong, mysterious and with a certain charm. The kidnapped character themselves can also be really popular as readers follow their journey of survival and adaptation in the vampire world.
The vampire lead, often named something like Alexander or Damian, is usually quite popular. He's the brooding, mysterious type with a tragic past. Then there's the female lead, perhaps a human named Isabella or Sophia. She's brave and curious, which makes her a great counterpart to the vampire. Another popular character could be the vampire's mentor, an older and wiser vampire who imparts knowledge about the 'Dance of the Vampires' and vampire lore in general.
Issei is extremely popular in highschool dxd fanfiction. He's the main character in the original series, and fanfiction writers often like to expand on his adventures, his relationships, and his growth as a character. They might give him new powers or put him in different situations to see how he'll react.
Well, the main character, let's say a young vampire named Alex, is often very popular. He's usually brave and curious, always getting into adventures around the school. Then there's the headmaster, who is a very old and wise vampire. His character is often explored in fanfiction as he has a lot of secrets and knowledge about the vampire world.
Naruto himself is often a very popular character. His unique abilities and his growth - from an outcast in his village to a hero - make him a central figure. People like to see how he would interact with the DxD world.
Alucard from Hellsing is often a popular choice. His powerful and mysterious nature makes him stand out. In the context of a crossover fanfiction, seeing how he would interact with the characters of Highschool DxD, especially the devils and angels, is really exciting for readers.
Akeno is another character that gets a lot of attention in these fanfictions. Her dual - nature of being sweet and sadistic makes for some great character development in the VRMMO world. Maybe she uses her electrical powers in unique ways to gain an edge in the game.
Aizen himself is often a very popular character in these fanfictions. His powerful and mysterious nature makes him a central figure that readers are interested in seeing interact with the Highschool DxD cast. Another popular character is often Issei, as his growth and development in the original series makes him an interesting counterpart to Aizen. Their interactions can range from adversarial to a strange sort of mentorship.
Naruto himself is often very popular. His energetic and determined personality makes him stand out. He can bring his ninja skills and positive attitude into the Highschool DxD setting. Also, Issei from Highschool DxD is commonly included because of his central role in his own series and his unique relationship with the devils and other supernatural beings.
Issei Hyoudou is often a popular character in such fanfiction. Since he's the main character in Highschool DxD, he's likely to be at the center of any crossovers. His determination and growth throughout the original series make him an interesting character to see interact with the Slender Man concept.