Another possibility is that an ancient and evil magical artifact is the real antagonist. It has the power to influence Lily's thoughts and actions, and it compels her to kill Harry in order to fulfill some dark prophecy or purpose related to the artifact's power.
It could be a dark wizard who is jealous of the bond between Harry and Lily. This wizard might try to break their connection and use Lily's power for his own evil purposes. Maybe he wants to control Lily's magic, which is known to be strong, to achieve domination over the wizarding world.
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.
It could be a dark wizard who is trying to prevent Lily and James from meeting Harry. This wizard might be a follower of Voldemort who is still bitter about the downfall of their master and wants to disrupt this family reunion to cause chaos in the wizarding world.
In the fanfic, Lily's fairy heritage could make her a target for dark forces. Harry, being the hero he is, would then vow to protect her. Their relationship would grow as they face numerous challenges together, and Harry might learn some new spells or techniques from Lily's fairy magic along the way. This would not only strengthen their bond but also make Harry a more powerful wizard.
One possible plot could be that Lily is under some dark magic influence which makes her turn against Harry. Maybe a powerful dark wizard has corrupted her mind, leading her to believe that Harry is a threat to the wizarding world that needs to be eliminated.
It could be a dark wizard who wants to prevent Ginny from meeting James and Lily. Maybe this wizard has some grudge against the Potter family and knows that if Ginny meets them and learns something important, it will foil his evil plans.
Perhaps in this fanfic, Harry has been influenced by dark magic or some evil entity. This external influence could distort his perception of Lily and turn his normal feelings for her into hatred. Dark magic has the power to corrupt one's mind and change relationships in unexpected ways.
Obviously, Harry and Lily are the main characters.
One great fanfic could be 'Harry and Lily's Enchanted Bond'. In this story, Lily, being a fairy, might use her magical fairy abilities to help Harry in his adventures at Hogwarts. It could add a whole new dimension to the wizarding world as we know it.
Lily could be an ordinary Pokemon fan who has a special love for fan - made Pokemon stories. Maybe she's a young girl who has been into Pokemon for a long time and has discovered the world of fanfic as a way to expand her love for the franchise. Or she could be a more prominent figure within a specific Pokemon fan community, known for her enthusiasm for fanfic.