Maybe it's some kind of special jutsu or a curse that backfires in a way that numbs his pain receptors. For example, a powerful sealing technique might have been placed on him accidentally or on purpose that has this side - effect.
His fighting style could become more aggressive. Without the fear of pain, he might be willing to use more extreme techniques that would otherwise be too dangerous because of the pain they cause to the user. He could push his body to the limit more often.
The plot could involve Minato being brought back to life or having a vision where he steps into Naruto's shoes. He might feel Naruto's pain from constantly being underestimated because of his status as a Jinchuriki. This could lead to Minato trying to find ways to comfort Naruto from afar, or even influencing the events around Naruto in a supernatural way.
He could feel it through a shared chakra link. Since they are related and Minato sealed the Nine - Tails in Naruto, there might be a latent connection that gets activated in the fanfiction, allowing him to sense Naruto's pain on a physical and emotional level.
I'm not sure specifically as there could be many different interpretations. It might be about Harry from the Harry Potter series reliving or coming to terms with past traumas, like the loss of his parents or the challenges he faced at Hogwarts in the past.
I'm not sure specifically as there could be many different takes on it. It might be about a character named Percy who is haunted by past traumas or difficult experiences. Maybe it delves into his emotional journey as he tries to come to terms with what has happened before.
There's no way to tell who the author is just from the title 'percy feels the pain of the past fanfiction'. It could be written by a relatively unknown fanfiction writer or someone who just started writing in the fandom. There are countless fanfiction platforms, and it could be on any one of them, making it difficult to track down the author without more details.
In many fanfictions, Naruto might use new jutsus or a deeper understanding of his own power. Maybe he taps into a more powerful form of the Nine - Tails' chakra and combines it with his Sage Mode in a unique way to overpower Pain.
In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.