Obviously Finn himself is a central character. Jake, his best friend, is also very likely to be included. Princess Bubblegum often shows up as she has a complex relationship with Finn. Marceline might also be in there, as she is an important part of Finn's world and can play a role in his growth and maturity.
Finn is the main focus, of course. Jake is always around in these fanfictions, as they are a duo. Princess Bubblegum usually plays a part, especially when it comes to Finn's growth in areas like leadership and understanding complex relationships. Marceline the Vampire Queen is another popular character. Her unique personality and her past can add depth to Finn's journey of maturing, for example, through the stories they share.
Finn is at the heart of it. Jake is essential as his constant companion and source of support. Princess Bubblegum, with her scientific knowledge and her role as a leader, often appears. She can influence Finn's development in many ways. Marceline is popular too. Her laid - back yet deep - thinking nature can contrast with Finn's more impulsive self, and their interactions can be a great part of his maturing process. There may also be other characters like BMO, who can add a touch of fun and innocence to the story while Finn is on his path to maturity.
Some common themes could be Finn's growth in terms of his relationships. For example, his friendships might deepen or change as he matures. Another theme could be his self - discovery, as he figures out who he is becoming as he gets older. There might also be themes related to his role in the world he lives in, like how his responsibilities increase as he matures.
Shepard is a very common character to be at the center of a human harem in fanfiction. Given their heroic status and the fact that they interact with so many different species, it's easy to imagine a harem forming around them.
Obviously, Batman himself is the central character. His complex personality and the situation of being unmasked provide a lot of material for fanfiction writers.
Amy Rose is often included. She has a long - standing crush on Sonic, so she's a natural fit for the harem concept. Tails might also be included in some more unique takes on the harem, perhaps in a platonic way where he's part of Sonic's support group within the harem. Cream the Rabbit could also be a character in sonic harem fanfiction, adding a cute and innocent element to the harem.
Obviously, Ben Tennyson is the most popular character. He is the main character in the Ben 10 series, and in the gamer fanfiction, he is often the center of the story, using his Omnitrix to fight various enemies. Another popular character is Gwen. Her magical abilities add an interesting dynamic, especially when combined with Ben's alien powers in the context of a gamer - inspired adventure. Also, Grandpa Max is often included as he provides guidance and wisdom to Ben and Gwen during their quests.
Obviously, The Flash himself is a central character. Then there's Iris West, his love interest. Cisco Ramon, with his technological expertise, often plays an important role too. Also, Caitlin Snow, who can transform into Killer Frost, is a popular character in these fanfictions.
Ron Weasley is also popular. He could be a source of comic relief as well as a loyal friend in the new story. If Harry is reborn into a different family or situation, Ron might be one of the first to notice the changes in Harry and help him blend in or deal with any difficulties that come his way.
Shepard is often a central character in Mass Effect harem fanfiction. Given their role as a hero in the main games, it's easy to imagine them in a harem situation. Liara is also very popular. Her connection with Shepard in the original story and her asari abilities make her an interesting addition to any harem. Garrus is another one. His loyalty and unique turian characteristics can add a lot to the harem dynamic.
Naruto himself can be considered as well. His Uzumaki bloodline is very powerful. In a harem fanfiction, his presence can attract characters with different bloodlines. His own journey and growth can also be intertwined with the exploration of the harem members' bloodlines and their relationships.
Sasuke is often included. In the genderbend concept, his relationship with the female Naruto can be a central focus. He might be the brooding, mysterious type who is drawn to the female Naruto's unique charm. Kakashi is also popular. His cool and collected demeanor might add an interesting dynamic to the harem, perhaps as a mentor - like figure who also has some underlying romantic feelings.