Reacting to this title, first, we should recognize that the use of the word 'bastard' is inappropriate. We can choose to ignore it if we don't want to engage with offensive content. However, if it's part of a community where respect and positive values are promoted, it could be a good idea to let the creator know that such a title is not acceptable. This could be a chance to educate about the importance of respectful language in relation to fictional works and their characters.
Using the term 'bastard' in this context is inappropriate and disrespectful. 'Kushina' is Naruto's mother in the Naruto series. A 'fanfic' is a fan - created fictional work. But this title seems to convey a negative and unkind sentiment towards a character or a work related to Naruto.
At first, Naruto would be shocked and confused. His world view would be shattered. But then, he would draw on his inner strength. He has overcome so many hardships before. He might feel a deep sadness, but also a sense of resolve. He would still believe in the power of love and friendship, and use that to face Kushina's betrayal. He could also use this experience to grow even stronger and more determined to prove himself and his beliefs.
Naruto would be heartbroken. He might initially be in denial, not believing that his mother could forget him. Then he would probably get angry, vowing to find out who or what caused this and make them pay. After that, he'd turn his anger into determination and start looking for solutions to bring back her memories.
Iruka would probably be one of the first to notice and try to step in. He has always been like a father figure to Naruto. He would likely confront Kushina and try to protect Naruto. Sasuke might be a bit more conflicted at first. He has his own issues, but seeing Naruto being abused, he would probably end up on Naruto's side and try to figure out what's wrong with Kushina.
At first, Naruto would probably be extremely confused and hurt. He loves his mother and can't understand why she would do such a thing. He might try to find her immediately to talk it out.
Naruto might first be in denial. He would find it hard to believe that his mother would break a promise to him. But as the truth sinks in, he could become angry. He might lash out, either physically or verbally, at those around him who remind him of the broken promise. Eventually, he would probably turn this pain into motivation to prove himself despite the setback.
Kushina would be extremely worried. She would rush to Naruto's side immediately and start using her medical knowledge or any available resources to help him get better.
She might be devastated. She could break down in tears immediately.
They would be absolutely devastated. Minato, as Naruto's father, would likely be filled with guilt for not being there to protect him. Kushina, as his mother, would be heartbroken. They might first be in a state of shock, unable to believe what has happened. Minato might blame himself for leaving Naruto alone in a world full of dangers. Kushina would probably hold Naruto's body close, crying uncontrollably, and their grief could be so overwhelming that it would change the entire course of the ninja world they live in as their connection to Naruto was a major part of their lives.
Naruto might be extremely heartbroken. He has always looked up to his mother, and being disowned by her would be a huge blow. He could become withdrawn and start to question his own worth.