Who could be the main antagonist in Danny Phantom twin sister fanfiction?
The main antagonist could be a group of human hunters who are after Danny and his sister because they think all half - ghosts are dangerous. They might be more organized and have more advanced technology than the usual ghost hunters in the show. They could be constantly chasing the twins, making it difficult for them to have a normal life while also trying to protect the town from other ghosts. This would create a lot of tension as the twins have to deal with both human and ghost threats.
One possible main antagonist could be a powerful ghost who sees the combination of Danny and his twin sister's powers as a threat. This ghost could be trying to split them up or turn them against each other so that they can't use their powers together to stop his evil plans. Maybe he has some connection to their family's past in the Ghost Zone that makes him particularly interested in them.
A really interesting main antagonist could be another half - ghost who is jealous of Danny and his sister. This half - ghost was shunned in the Ghost Zone for not being 'pure' enough or something similar. Now, he wants to prove that he is more powerful than them by causing chaos in their town and trying to defeat them in battle. He could be very sneaky and manipulative, trying to turn their friends against them and causing all sorts of problems in their personal lives as well as their ghost - fighting duties.