I'm not sure specifically as we don't have more details about this particular 'the pain of love novel'. But often in such novels, there is a male and a female lead. The male lead could be a brooding, complex character who has his own demons to fight.
I'm not entirely sure as I haven't read it specifically. But generally, a graphic novel with such a title might be about a character or characters seeking vengeance in a very intense and vocal way. It could involve themes of justice, revenge, and the journey to right some wrongs.
In a 'naruto after pain fanfic', Naruto is key. However, characters like Tsunade are important too. She can lead the medical and administrative aspects of the village's recovery. Then there are Naruto's friends like Kiba and Choji. They can help with the physical tasks of rebuilding, and also in the security patrols around the village to prevent any new threats.
It's hard to say without actually reading the fanfiction. But it could be the classic pairing of a male and female lead. For example, in a Twilight - inspired fanfic, it might be Edward and Bella where Edward kisses Bella's pain away. Or it could be two best friends who have a deeper connection than they realize at first.
I don't really know who the main characters are since I haven't read or watched it. But they could be patients, doctors, or people from pharmaceutical companies who are involved in the pain - related activities.