Who is the ghost girl in Saga?

Could you please clarify who the "ghost girl" in Saga refers to? Is she a recurring character throughout the series, or does she only appear in a specific arc? If she's a key figure, what role does she play in the story? Is she a mysterious figure shrouded in secrecy, or does she have a more tangible presence? Additionally, is there any significance to her being a "ghost," or is it simply a descriptive term used to evoke a certain atmosphere? I'm eager to learn more about this intriguing character and her place in the Saga universe.