Luna could be another main character. Given her unique perspective on things, she might offer Ginny some unusual but very helpful advice about the pregnancy. And she could be there when Ginny faints, providing immediate assistance or at least a calming presence.
Obviously Ginny would be one of the main characters since she's the one who faints and is pregnant. Harry could also be a main character as he might be her partner or husband, and he would be involved in the story, supporting Ginny through the pregnancy.
Hermione could also be a main character. She could be Ginny's confidante, the person Ginny turns to for advice when she discovers she's pregnant. Hermione's intelligence and practical nature could play an important role in helping Ginny deal with the situation.
One possible plot could be that Ginny faints due to the shock of finding out she's pregnant and then has to deal with the reactions of her family and friends. They might be initially surprised but then offer support.
The main characters are Ginny and Blaise, of course. Ginny, known for her feisty and brave nature from the Gryffindor house. And Blaise, who is often depicted as a rather mysterious and somewhat aloof Slytherin. In this fanfiction concept, they are the central figures around whom the whole story of the unexpected pregnancy revolves.
In some fanfictions, there could be a new dark wizard or witch emerging. They see Harry and Ginny's unborn baby as a potential threat to their future plans of taking over the wizarding world. So, they start plotting to harm Ginny or the baby. This new antagonist could have a group of followers who help them carry out their devious plans, making Harry and Ginny's pregnancy a time of great danger and uncertainty.
Also, members of the Weasley family like Molly and Arthur Weasley are likely to be key characters. Molly would be all excited and involved in the pregnancy, giving Ginny all sorts of motherly advice and care. Arthur might be interested in how magic could be involved in the development of the baby, as he is always fascinated by the wizarding world's wonders.
Most likely, Tony Stark himself is a main character as the story seems to revolve around him fainting while being sick. Pepper Potts would probably be another important character as she is often by Tony's side in these situations. And the other Avengers like Steve Rogers, Natasha Romanoff, etc. could also be involved as they are his friends and teammates.
Obviously, Harry would be an important character even though he leaves. His actions before leaving and potential return would greatly affect the story. Also, the Weasley family, especially Ginny's mother Molly. Molly would probably be very involved, offering her motherly advice and support. Ron and Hermione might also be involved. Ron as Ginny's brother would surely have an opinion and Hermione could be there for Ginny as a friend.
Hermione could be a main character. She would use her vast knowledge of magic to try and find a cure or treatment for Ginny's injury. Luna Lovegood could also be important. She might have some unique insights or magical remedies that could help Ginny. And of course, Ginny's parents, Arthur and Molly Weasley, would be main characters. They would be extremely worried and do everything in their power to take care of their daughter.
There could be the tattoo artist. He or she might be an interesting character with a lot of stories about the tattoos they've done in the past. They could have a unique personality and play an important role in guiding Ginny through the process.
It could be a long - time boyfriend that we haven't known much about in the original story. Maybe he's a character who was in the background but now becomes central to the plot.
Another possibility is that it could be an original character created for the fanfiction. Maybe some new boy at Hogwarts who Ginny fell for and had a secret relationship with, resulting in the pregnancy.