A believable OC should have relationships. They could have a best friend from their human days if they're a vampire or witch now. In relation to Rebekah, their initial reaction to her should be realistic. If they're a normal human and meet Rebekah, they might be scared at first. Their skills and abilities should also make sense. If they're a fighter, show how they learned their combat skills. For instance, if they were trained by a werewolf pack in secret, that adds an interesting layer to their character.
To create a believable OC, start with a clear backstory. For example, if the OC is a vampire, explain how they were turned. If they're a witch, detail their family's magical lineage. Their personality should also be well - defined. Are they brave or cautious? Kind or cunning? Also, give them goals and motives that fit within the world of Rebekah Mikaelson. Maybe they want to find a cure for vampirism, or they're seeking revenge on an old enemy.