A school nurse could also be a main character. In the fanfiction, Lily could go to the nurse when she has cramps or has questions about her period, and the nurse could give her professional medical advice and resources.
Her best friend could be a main character. This friend might be the first one Lily tells about getting her period and could offer practical advice like which pads are the most comfortable.
It could be about Lily's friends supporting her during this new experience. Maybe they bring her comfort items like warm tea and chocolate.
Perhaps Sirius Black could also be involved. Sirius was Harry's godfather and was very much a part of Harry's life. His presence could add some interesting dynamics, like he could be the one who stumbles upon the magic that makes the meeting possible and then has to deal with the implications of Harry meeting his mother.
I think Hinata would be a great supporting character. Hinata is very empathetic and kind. She would be there to listen to Sakura's concerns and offer her quiet support. Also, Kakashi could play an interesting role. He might not fully understand what's going on but still offer his general wisdom and support to Sakura in his own unique, laid - back way.
Percy would probably be an important character. He's Annabeth's close friend and love interest. His reaction to Annabeth getting her period could be interesting, like he tries to be supportive but is a bit clumsy at first.
Obviously, Lily is one of the main characters. And her friends, though we don't know their names yet from just this title, are also main characters. They are likely to be students at Hogwarts with different personalities and magical abilities.
The main characters are Darkness and Lily. Darkness could be a complex character, maybe having some hidden motives or a tragic past that makes it act in certain ways. Lily, on the other hand, might be the one who tries to understand Darkness and bring out the good in it. She could be kind - hearted and have a strong sense of justice.
Harry Potter could be a main character. If he's the father, he would play a significant role in the girl's life, teaching her about magic and protecting her. He could also pass on his own experiences and values.
Sirius Black might be involved. As Harry's godfather and James' friend, he might have some knowledge about the trunk. He could help Harry understand the significance of the items in it. Also, Dumbledore could be involved. He always seemed to know a great deal about everything, and he might have some information about Lily's trunk that could help Harry on his journey of discovery.
It might be because Snape said or did something hurtful to Lily's friends. Lily is a loyal person, and if Snape insulted or harmed her friends in some way, she would be furious enough to send a Howler. In the context of fanfiction, there could be a new character introduced who Snape treats badly, and this angers Lily.
It could be her best friend. Her best friend might have been looking for her when she heard Lily's cry of pain. Without hesitation, the best friend would run towards the sound and find Lily. Then, using whatever magic or physical strength they have, they would get Lily to safety and call for help.