They might start off as enemies. Sirius doesn't get along with his family, so he may not want the Marauders to have anything to do with Regulus. But sometimes, they could end up working together for a common good.
Sirius would probably be angry at first in the fanfiction. He left his family and Regulus was still part of it. But as the story progresses, he might start to feel some regret or longing for his brother. James would be supportive of Sirius' emotions, and Remus would be the voice of reason, analyzing Regulus' actions and motives.
There are fanfictions where they meet in a dangerous situation. For example, in a Forbidden Forest encounter. The Marauders are exploring, and Regulus has gotten lost. They initially suspect each other of ill intentions but then realize they need to work together to get out. And there's also the type where they meet at a pure - blood event, which challenges their different beliefs about blood purity and friendship.
Archive of Our Own (AO3) is a great platform for 'Marauders meet Regulus' fanfiction. It has a large collection with various interpretations of their meetings. Fanfiction.net is also a well - known platform where you can find such stories. There are different tags and filters to help you find exactly what you're looking for.
They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
Regulus Black was a member of the Black family. He was initially a Death Eater, following in the footsteps of Voldemort. However, he later had a change of heart. He discovered Voldemort's Horcruxes and tried to destroy one. He left a note for Kreacher, his house - elf, asking Kreacher to lead Harry Potter to the locket Horcrux. His actions showed that he was brave in the end, trying to make up for his past mistakes in following the Dark Lord.
The marauders' reaction would be quite complex. They might feel a sense of nostalgia as they read about the world they once knew, but through a different lens in the fanfiction. They could also be a bit critical. If the fanfiction didn't seem to respect the essence of the original story, they might be quick to point it out. However, if it was well - written and added new depth to the story, they could end up really enjoying it and even discussing it at length among themselves.
In some fanfictions, Regulus Black's involvement in Quidditch could be linked to his family's expectations. The Black family might have certain standards for him in the sport, either pushing him to excel or causing him internal conflict as he tries to balance his own love for the game with their demands.
Regulus Black was a complex character. He was once a Death Eater but later had a change of heart. He discovered Voldemort's Horcrux secret and tried to destroy one. He sacrificed himself in the process. His story shows that people can break free from the dark path they initially chose.