Fluttershy could be an interesting character to include. Her gentle nature could be a contrast to Shadow's more brooding personality. It could lead to situations where Shadow learns to be more empathetic or Fluttershy becomes more assertive in his presence. Sunset Shimmer is also a key character as she has a complex backstory and her knowledge of both worlds can play a significant role in a fanfiction involving Shadow.
Twilight Sparkle is really popular. Since she's the main character in the original series, a lot of fanfictions revolve around her adventures and her trying to adapt to the human world in the Equestria Girls universe.
Another often - seen pairing is Sakura and Twilight Sparkle. Sakura is a strong - willed and intelligent character in Naruto, and Twilight is the same in Equestria Girls. They could bond over their love for knowledge, with Sakura sharing her medical ninjutsu knowledge and Twilight sharing her magical research. Also, they could support each other in facing challenges that require both physical and magical skills.
Sora is often a very popular character. He's the main protagonist in Kingdom Hearts, and when he's brought into the Equestria Girls' world, his heroic nature and his skills with the keyblade make for exciting storylines. Twilight Sparkle is also popular. She's a powerful magic user in the Equestria Girls universe, and her interactions with Sora or other Kingdom Hearts characters can be really interesting. Another popular character is Aqua. Her grace and combat abilities can add a new dimension to the fanfiction when she teams up with the Equestria Girls.
Twilight Sparkle is often a popular character. Her intelligent and curious nature makes for great reactions in these fanfics. She's likely to analyze situations deeply and come up with plans.
Obviously, Blossom, Bubbles, and Buttercup are the most popular. They are the main characters after all. But villains like Mojo Jojo are also often included as they provide great conflict. Also, the Professor who created the Powerpuff Girls can be an important character in fanfiction, especially if it involves the origin story or some kind of scientific experiment gone wrong.
Professor Utonium is a popular character to include. He is the creator of the Powerpuff Girls and his relationship with them can be explored in different ways in fanfiction. Maybe he creates a new experiment that has unexpected consequences for the girls. Another popular character could be the Rowdyruff Boys as they can create interesting rivalries and storylines with the Powerpuff Girls.
Well, Spike and Big McIntosh are also characters that some fanfics like to swap. Spike is small and often in the background but has his own charm, while Big McIntosh is a large, strong and mostly silent character. Swapping them can lead to some really unexpected situations. For example, Spike in Big McIntosh's body would have to deal with the physical strength and the work on the farm. And Big McIntosh in Spike's body would have to navigate the more magical and often chaotic situations that Spike is used to.
Twilight Sparkle from My Little Pony is also quite popular in these crossovers. Her intelligence and powerful magic make her a great counterpart to Naruto. She can have really interesting interactions with Naruto, whether it's about sharing knowledge or working together in battles.
Celestia is often a popular character. She's a powerful and regal figure, and in fanfiction, her leadership during the war can be a central focus. Another popular one is Twilight Sparkle. Her intelligence and magical abilities make her a key player in many war - related scenarios. Rainbow Dash also tends to be popular as she's brave and her flying skills can be useful in battle.
Well, aside from Shadow, Omega is a really cool character to include. His robotic nature and his loyalty to Shadow can create some interesting storylines. Tails could also be included, especially if you want to explore the scientific side of the Sonic universe in your fanfiction. He can interact with Shadow in various ways, like helping him with some sort of technological problem or even getting into an argument about the ethics of certain actions. Another popular character is Knuckles. His guardian role of the Master Emerald can bring in elements of mystery and adventure when combined with Shadow's story.