Sonic, without a doubt, is the most crucial character. But in addition to him, characters like Silver can bring a different perspective. His ability to manipulate time in a way could be incorporated into the Black Knight story in unique ways. Also, Rouge. Her thieving skills and her relationships with other characters can be a great addition. And don't forget about the characters native to the Black Knight world, like King Arthur - like figures, who can add a touch of medieval fantasy to the fanfiction.
Harvey Dent, or Two - Face, is another favorite. His fall from grace and his split - personality concept offer a lot of room for exploration. Fanfiction can delve into his recovery attempts or his further descent into villainy.
Definitely Sonic. He's the star of the show. Then there's Amy. She can bring in the element of love and determination. Shadow could also be a great addition. His brooding and powerful nature can create some interesting storylines, especially if he has to team up with Sonic in the Lost World. And don't forget about Cream and Cheese. Their cute and innocent presence can contrast with the more dangerous aspects of the Lost World.
In sonic the hedgehog crossover fanfiction, Sonic himself is of course the main character. But other characters that are popular to include are Amy Rose. Her love for Sonic can create interesting relationship dynamics in crossovers. Another popular one is Dr. Eggman. His villainous nature can be a great source of conflict in crossovers with other characters. And then there are characters from other video game series that are also quite popular, like Link from The Legend of Zelda. His use of weapons and exploration skills could make for some really exciting crossovers with Sonic.
Definitely Joker. He's such an iconic character in The Dark Knight universe, and when it comes to crossovers, his chaotic nature can create great interactions with characters from other franchises. For example, if he meets the Joker from the DC Animated Universe, it could be a really wild and unpredictable encounter.
Tails is a very popular character to include. He is Sonic's best friend and his technical skills can add a lot to the story. For example, he could help Sonic and Shadow figure out some complex machinery or technology that is part of the plot.
Mario and Sonic themselves are the most obvious and popular choices. They are the main characters of their respective franchises and their rivalry is always exciting to explore.
Princess Peach from the Mario universe is often included. She can add a touch of elegance and sometimes be a damsel in distress or a helpful ally. Tails from Sonic's world is also popular. His intelligence and flying ability can be useful in various situations in the fanfiction, like helping Sonic and Mario solve puzzles or reach high places during the Olympic events.
Bowser can also be a great addition. He can be the antagonist who tries to cheat in the Olympics or cause trouble for Sonic and Mario. And from Sonic's side, Knuckles is a fan - favorite. His strength and gliding ability can make for some interesting interactions with the Mario characters. For example, he might get into a friendly competition with Donkey Kong over who is stronger.
The main characters are Sonic, of course. He's the fast - running hero who gets drawn into the world of the Black Knight. And Merlina, who has her own motives and powers within this world.
Sonic, of course, is a must - have. But also Knuckles from both universes. In Sonic Boom, Knuckles is a bit more of a muscle - brained character in a fun way, while the classic Knuckles is more serious about guarding the Master Emerald. When they meet, there could be some hilarious misunderstandings at first, like Sonic Boom Knuckles trying to show off his strength to the other Knuckles, but then they end up teaming up against a common threat.