Maybe a house - elf who has broken free from the traditional servitude mindset. This house - elf has developed their own magic in a different way and befriends Harry. They teach Harry about the hidden magic that house - elves possess, which is not taught in regular Hogwarts classes.
Perhaps Harry discovers a group of magical creatures that can communicate with him in a special way. These new friends introduce him to a whole new side of the magical world that he was previously unaware of. For example, they might show him hidden places where rare magical plants grow that can be used for powerful potions.
It could be an uncle. Maybe an uncle who was in hiding all this time because he was afraid of Voldemort. Now that Voldemort is gone, he comes out and reveals himself to Harry. He could have a lot of knowledge about the Potter family secrets and magic that he shares with Harry.
I'm not sure specifically which 'harry finds a pendent' fanfic you're referring to. There could be many different interpretations. It might be about Harry from the Harry Potter universe finding a pendant that has some sort of magical significance, like it could be a key to an ancient magic or have a connection to his family's past.
Harry could also go into a state of shock and disbelief. He loves Ginny deeply, and to find out she's cheating would shatter his world. He might start to question everything about their relationship, like all the memories they've shared together. And he could become very withdrawn, not wanting to talk to anyone, even his closest friends like Ron and Hermione. He would need time to process his emotions and figure out what to do next.
In the fanfic 'Malfoy's Revelation', Malfoy is paired with Harry for a project. As they spend time together, Malfoy notices some strange behaviors from Harry, like flinching at sudden movements. After some snooping around, he discovers that Harry has been abused. This new knowledge makes Malfoy more empathetic towards Harry and he tries to help him in his own way, which includes standing up for Harry against some of their mutual enemies at Hogwarts.
I'm not sure specifically as I haven't read this particular fanfic. But generally, it might be about Harry Potter's discovery of a grimoire, which could be filled with spells, magical knowledge, or secrets related to the Potter family or Hogwarts magic.
She could start by joining new study groups or clubs at Hogwarts. Since she's so intelligent, she might find others with similar interests in advanced magic studies or ancient runes.
In such a fanfic, Snape might initially be shocked. Given his complex relationship with Harry in the original series, he could struggle between his old grudges and his sense of duty. He might take it upon himself to protect Harry secretly at first, perhaps by using his position at Hogwarts to keep an eye on Harry's abusers. For example, if it's the Dursleys abusing Harry, Snape could find ways to make sure they don't go too far without revealing his true intentions.
In such a fanfic, perhaps Ginny has been feeling neglected by Harry due to his constant focus on fighting dark forces or some other important matters. So she starts to seek comfort in someone else. When Harry finds out, he first feels angry and betrayed. But then he realizes that he also had a part to play in their relationship going sour. He could then try to win Ginny back or decide to move on, depending on how the author wants to develop the story.