Maybe the main antagonist is a scientist who has been experimenting with vampire-like powers and accidentally creates a powerful vampire - like monster. This monster then starts attacking the world, and Shadow, as a new vampire - hedgehog, has to stop it. The scientist might also be trying to cover up his mistake and will stop at nothing to keep Shadow from interfering.
Another option for the main antagonist could be a group of vampire hunters who mistake Shadow for a true, evil vampire. They constantly hunt him down, which forces Shadow to fight back. However, this also draws the attention of real vampires who see Shadow as a way to get revenge on the hunters, and Shadow has to deal with both threats.
Obviously, Shadow the Hedgehog is a main character. He's the one with the vampire - like qualities. Sonic could also be a main character in some fanfictions. He might be in conflict with Shadow because of his new vampire nature or could be trying to help him control it. Amy could be another main character, perhaps she has a special connection to Shadow and tries to understand his transformation.
The main antagonist could be an evil sorcerer who has corrupted the Black Knight and is using him to carry out his dark plans. This sorcerer could be after Sonic's speed power to enhance his own magic.
It could be another vampire clan. They might see Cartman and Kyle's relationship as a threat to their own power or way of life. For example, if Cartman and Kyle are trying to promote co - existence between vampires and humans, the other clan might oppose it.
Obviously, Shadow himself would be a main character even in his baby form. Sonic would also be a key character as he is often the hero in these Sonic - related stories. He would likely be involved in trying to figure out how to turn Shadow back. Tails could be another main character as his intelligence would be useful in finding a solution to Shadow's transformation. He might study the cause of it and try to come up with a remedy.
Perhaps a half - alien, half - vampire hybrid. This character was shunned by both species and blames Ben for making him feel even more like an outcast. He uses his unique powers to create chaos and tries to turn Ben into a vampire - alien like himself.
Perhaps a powerful vampire lord who sees Sookie as a threat to his power. Since she was once human and has connections to both worlds, he might fear that she could rally other vampires and humans against him. He could try to eliminate her by any means necessary, leading to a lot of action - packed scenes in the fanfiction.
It could be an old and powerful vampire who sees Hermione as a threat to his rule. Since Hermione is a very intelligent witch - now vampire - she might be able to figure out a way to undermine his power.
It could be an old and powerful vampire lord who sees Natsu as a threat to his rule. Since Natsu is a new and different kind of vampire, the lord might want to eliminate him.
It could be a powerful vampire clan that doesn't want Harry, a new vampire, to disrupt their ways. They might try to either kill him or force him to join their ranks against his will. They could see him as an outsider who doesn't belong in the vampire world and a potential danger to their secrecy.
Perhaps a vampire scientist who is conducting unethical experiments on both humans and vampires. Ichigo gets in his way when he discovers what the scientist is doing. This antagonist could be using his scientific knowledge to create powerful and dangerous hybrids. He believes that he is creating a new and better vampire - human world order, but his methods are causing a lot of harm, and Ichigo is determined to stop him.