Often, it's portrayed as a relationship full of tension. Harry is the hero of the wizarding world who fought against Voldemort, and Narcissa was on the side of the Death Eaters. But in fanfic, this can be flipped. Some portray Narcissa as secretly admiring Harry's courage, which could lead to a strange, unspoken respect between them.
The portrayal of their relationship in fanfic is diverse. It could be a relationship based on secrets. Maybe Harry discovers something about Narcissa that she wants to keep hidden, and this creates a bond of sorts between them. Or perhaps Narcissa, seeing Harry as a symbol of hope for the wizarding world, decides to support him from the shadows, leading to a relationship that is not openly acknowledged but still significant in the context of the fanfic's plot.
In some fanfics, Harry and Narcissa might have a complex relationship. It could start from a place of mutual wariness given their different allegiances in the original story. However, some fanfic writers might explore a more nuanced relationship where they find common ground, perhaps in their love for Draco. Narcissa's maternal instincts could lead her to see Harry in a different light over time, especially if she realizes his role in saving Draco.
They can be portrayed as a very traditional couple. Lucius as the dominant figure, making important family decisions, while Narcissa is more focused on taking care of Draco. But they both have a deep respect for each other's roles in the family.
There are also fanfictions that take a completely different turn and show a sort of begrudging respect between them. After a series of events where they both learn more about each other's pasts and motives, they start to respect the other's power and tenacity. This might not lead to friendship, but it does create a more nuanced relationship than just pure hatred.
In many Harry Potter fanfictions, Narcissa is often portrayed as a complex character. Sometimes she is shown as a mother who is desperate to protect her son, Draco. This is in line with her actions in the original series where she lied to Voldemort about Harry being dead to save Draco.
The relationship can be shown as one of deep trust. Ginny feeling comfortable enough to fall asleep on Harry indicates she trusts him completely. It could also show a sense of intimacy, like they are in a very close and personal space.
In some fanfictions, it starts with distrust. Harry sees Narcissa as part of the Malfoy family, which has been on the wrong side. But as the story progresses, they might find common ground, like protecting the wizarding world from a new threat. This makes them slowly build a reluctant partnership.
Often, it can be portrayed as an unexpected and somewhat unorthodox relationship. Narcissa, being from a different background and generation, her possessiveness might stem from seeing Hermione's unique qualities. It could be shown as a sort of mentorship - like possessiveness, where Narcissa wants to guide Hermione in the ways of the wizarding world she knows.
The relationship can also be shown as very loving. Poseidon might secretly watch over Percy and be proud of his son's achievements. He could send signs of his support during Percy's battles or quests. And Percy, in turn, is always eager to make his father proud and show that he is a worthy son of Poseidon.
At first, it would likely be very strained. Harry has a history of conflict with the Malfoys, and Narcissa is a Malfoy. But over time, they might start to communicate more. They could find that they both have a love for the wizarding world's history and start sharing stories. This could lead to a more amicable relationship.
In fanfiction, their soulmate relationship can be a journey of redemption for Snape. Harry, being the more forgiving one, helps Snape come to terms with his past. Their bond is painted as something that defies the odds. It might include scenarios where they protect each other in ways that surprise even themselves. They learn to trust each other completely, which is a big shift from their canon relationship.