Davina was a teenage witch who was supposed to be sacrificed in the Harvest and the show's protagonist, Klaus, saw her as a weapon to used against the witches, as a result, she had been the main subject of a power struggle between the werewolves, the witches and the vampires.
Marcel had kept her and used her as a weapon against the witches, however, he was trying to protect her from the Harvest ritual, where four teenage witches had to be sacrificed so that they can be with their ancestors.
Even though she was sacrificed, it failed as she was resurrected. She now had a lust of revenge against Klaus due to him hurting her friends and using her.
She did so by resurrecting his step-father, Mikael, the vampire who hunts vampires. However, she actually puts a spell on a bracelet that will have Mikael under her spell.
Davina was arguably an anti-villainess of the show as she wants vengeance on Klaus for torturing her best friend Joshua, killing her friend Tim and multiple attempts to kill her as he viewed her a threat to his reign. Davina can also be viewed as a protagonist as most of her actions were directed towards Klaus who despite being the show's protagonist had committed numerous crimes he deserved punishment for, and Davina tried to find the means to severe Klaus from his sire line as she didn't wish to kill all the vampires who'd die with him.