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.
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.
Often, Harry's knowledge of fighting in fanfiction comes from intense self-study and practice, as he realizes the importance of being well-prepared for the challenges ahead.
In many fanfictions, Naruto might start by training harder. He throws himself into his ninja training as a way to distract himself from the loneliness. It gives him a sense of purpose and also helps him become stronger.
He might focus on his studies. In the wizarding world, there are so many interesting magical subjects like Potions, Transfiguration. By burying himself in books and learning new spells, he can at least distract himself from the lack of friends.
Loneliness in Harry Potter fanfiction is often a central theme. It can be depicted through the inner thoughts of characters. For instance, Hermione might feel lonely when her friends don't understand her academic pursuits or when she faces prejudice for being a Muggle - born. Writers might show her sitting alone in the library, surrounded by books but still feeling isolated. Another way is by creating new situations where characters are separated from their loved ones and have to face challenges alone, highlighting their sense of loneliness.
Often, fanfiction depicts their interaction as intense. Harry, being the hero, is usually in opposition to Grindelwald's dark plans. For example, he might try to stop Grindelwald from gathering more followers or retrieving some powerful dark artifact.
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.
He might be very sad at first. Harry has always valued his friends, so being forgotten by them would be a huge blow.
In some fanfiction concepts, he might change his soul through powerful magic spells. Maybe he discovers an ancient spell in a hidden book in the Room of Requirement that has the power to transform his very essence, his soul.
In some fanfictions, Harry might hide his money in a secret compartment of his trunk. He could use magic to create a hidden space where no one else can find it.