One common plot device is the introduction of new characters who initially misunderstand Harry, which makes him feel more alone. For example, a new student might come to Hogwarts and, because of rumors or Harry's fame, avoid him or be unkind, adding to his sense of isolation. Another device could be the separation from his friends. Maybe there's a magical accident that sends Harry to a different dimension or a locked part of the castle where he's completely alone and has to figure out how to get back while feeling lonely and scared.
One way is by his body language. He could be described as always sitting alone in the common room, hunched over, not interacting with others. His eyes might be downcast, showing that he's lost in his own world of loneliness. Maybe he avoids eye contact, as if he's afraid of getting too close to people. The author could also use the setting to enhance this feeling. For instance, describing him walking alone in the empty corridors of Hogwarts at night, with only the sound of his own footsteps echoing, emphasizing his solitude.
In fanfiction, 'Harry Potter feels numb' could imply that he's in a state of shock or emotional overload. Maybe he's been through a particularly traumatic event, like seeing a loved one die or experiencing a great betrayal. This numbness could be his mind's way of protecting itself from the full brunt of the pain.
One way is through his relationships. For example, if he's been let down by his friends, he might start to feel numb towards them. He could start to distance himself and not feel the same emotions he once did. Another way is through his experiences with the wizarding world's politics. If he feels constantly used or manipulated, it could lead to this numbness.
One great 'Harry Potter feels betrayed' fanfiction is 'The Shattered Trust'. In this fic, Harry discovers that some of his closest friends have been hiding secrets from him that make him feel extremely betrayed. It delves deep into his emotional turmoil.
Hermione could still be a common character. Even in fanfiction where Harry is alone, Hermione might be mentioned in his thoughts or in flashbacks as someone who has influenced him. For example, he might recall her advice on how to study magic when he is facing a difficult spell alone.
One interesting story could be about Harry facing challenges alone in the Forbidden Forest. Maybe he discovers a hidden magical creature that no one else knew about and has to figure out how to deal with it on his own.
Well, it could be that the author wants to show Harry's perseverance and growth through constantly dealing with unwanted attention. It makes his character stronger.
One common theme is the shattering of relationships. When Harry feels betrayed, it often involves his friends or mentors. For example, in many fics, Ron's jealousy might lead to actions that make Harry feel betrayed, and this shows how fragile their friendship can be.
One reason could be that in this fanfiction, Snape has some hidden kindness towards Harry that he doesn't show in the original story. Maybe he secretly protects Harry from other threats, which makes Harry feel safe around him.