Harry might initially feel sad and confused. He could start to withdraw into himself, spending more time alone. Maybe he'll question what he did wrong to be treated this way.
Perhaps there has been a misunderstanding. Maybe Harry did something that was misinterpreted by his mates, leading to their neglect.
He might become withdrawn. Naruto, who was always so full of life and energy, could start to keep to himself more. He might stop seeking his family's approval and focus on his own path, whether it's through training or making new friends outside of the family.
Harry might become withdrawn and introverted. Feeling unloved and unwanted, he could start to keep to himself more, spending time alone in his room or exploring the grounds of their home alone, trying to make sense of why his parents seem to favor the twins.
He might become withdrawn. Feeling unloved and ignored, Harry could start to keep to himself more, not sharing his thoughts and feelings with his family. He may also become resentful towards the twins, seeing them as the cause of his being neglected, even if it's not entirely their fault.
He might be shocked at first. It's not every day you find out you have mates. He could be a bit clumsy around them, not knowing how to act. Maybe he stutters when he tries to talk to them or gets all flustered.
He might start to rely on his friends more. For example, Sasuke and Sakura could become his main source of emotional support. He would focus on training with them and sharing his feelings, which in turn helps him to grow stronger and more independent.
It could mean that Harry Potter fanfiction is being overlooked or not getting the attention it deserves. Maybe there are so many fanfictions out there that the really good ones about Harry Potter are lost in the crowd.
Harry typically copes in several ways. Firstly, he may turn to magic as a form of escapism. Using magic to create things or go on adventures on his own can help him deal with the neglect. Secondly, he might start to form a surrogate family with his close friends. They can provide the love and care that his biological family lacks. And finally, he could also pour his energy into achieving great things at school or in the wizarding world to prove his worth despite his family's neglect.
Well, in such fanfiction, Harry's neglect by his parents can cause him to have a deep - seated fear of abandonment in his friendships. This could lead to him constantly testing his friends' loyalty. On the other hand, his friends, if they are true friends, will be more patient with him and try to show him unconditional love, which in turn can strengthen their relationships in the long run.
Kurama might become angry at first. He's a powerful being and being neglected would likely wound his pride. He could start causing some minor disruptions inside Naruto, like making it harder for Naruto to access his chakra.