Sebastian is often a popular choice. His demonic nature and his loyalty (in a rather twisted way) to Ciel make him a very interesting character to cross over. He can easily interact with characters from other dark or supernatural works, and his powers and suave personality can add a lot to a crossover story.
Well, aside from the main trio of Charlie, Alastor, and Angel Dust, Husk is becoming a popular character to include. His grumpy and alcoholic nature can provide some comical relief in crossovers. Niffty is also a great addition. Her hyper - active and somewhat naive personality can be a contrast to more serious characters from other crossovers. And let's not forget Lucifer, who can bring a whole new level of power and mystery to the crossover fanfic.
Mario and Sonic themselves are obvious choices. They are the main characters of their respective series and their rivalry - turned - friendship can be a great focus. Luigi is also popular as Mario's brother, and Tails is a fan - favorite in the Sonic series. Tails' intelligence and his ability to fly can add an interesting dynamic to the story.
Sebastian Michaelis is extremely popular. His cool and composed demeanor, along with his loyalty (albeit in a rather complex way) to Ciel, makes him a fan favorite. He also has his demonic charm that draws readers in.
Definitely Venkman. His sarcastic and witty personality can create great interactions in crossovers. For instance, if he were in a crossover with the X - Men, his humorous take on the mutants' powers would be hilarious.
Hajime Nagumo from 'arifureta' is a popular choice. His overpowered abilities and his journey from being a weakling to a powerful hero make him an interesting character to put in crossovers. Yue is also very popular. Her magic and her relationship with Hajime can add a lot of depth to a crossover story. Another character is Shea. Her energetic and somewhat naive personality can create humorous and interesting interactions in a crossover.
Akagi is another fan - favorite. Her character has depth and mystery. In a crossover, she could be the key to unlocking some complex plotlines, perhaps she has a connection to a powerful artifact in another world or has some hidden knowledge that is crucial for the story.
In Ben 10 crossover fanfiction, many writers like to include Azmuth. As the creator of the Omnitrix, he has a wealth of knowledge and power. His presence can add a layer of depth to the story, especially when it comes to explaining the science behind the Omnitrix or helping Ben understand new alien forms. Also, Charmcaster is often included. Her magic abilities can create some interesting dynamics when combined with Ben's alien powers in a crossover.
Joker is definitely one of the most popular. His cool and mysterious demeanor makes him a great fit in various crossovers. Also, Morgana is often included as his sidekick. Another popular character is Makoto, with her strong - willed and intelligent personality.
Batman himself is obviously a must - include. His leadership and skills are central to the Batfamily dynamic. Also, Robin, especially Damian Wayne as his complex relationship with Batman adds a lot of drama. Nightwing is popular too, with his acrobatic skills and his own journey away from being Robin. Batgirl, like Barbara Gordon, is often included for her intelligence and fighting abilities. And Alfred, the ever - loyal butler, who often provides support and wisdom to the family.
John Connor is often a popular choice. His leadership skills and importance in the Terminator series make him a great character to bring into other fictional universes. For example, in a crossover with a superhero world, he could be seen as a strategist working with the heroes.