Not having more details about the 'back for blood story' makes it difficult to accurately name the main characters. However, in a general sense, it could be a wronged individual, perhaps a former slave who has escaped and is now 'back for blood' against his or her former masters. Or it could be a detective who has a personal vendetta against a serial killer and is 'back for blood' to bring them to justice. There might also be side characters like friends or family members of the main character who either support or try to stop their quest for revenge.
The main characters in the 'back for blood story' might include a vengeful hero or anti - hero. They could be a person with a tragic past, like a soldier who lost his comrades and is now back to fight the enemy that caused their deaths. There could also be a villain or a group of villains who are the target of this 'back for blood' quest. It really depends on the nature and genre of the story. For example, in a supernatural story, it could be a witch who was exiled and is now back for blood against the villagers who drove her out.
Since I don't know the specific 'back for blood story', I can't say for sure who the main characters are. But they could be a victim seeking revenge (the one 'back for blood') and the antagonist who wronged them in the first place.