In fanfiction, the Marauders interact with baby Harry in multiple ways. James, being Harry's father, is extremely affectionate. He showers Harry with hugs and kisses. Sirius, on the other hand, is the fun - loving uncle type. He might take Harry on little broom rides around the house (using magic to keep him safe of course). Remus is the gentle storyteller. He shares tales of magical creatures and ancient spells with Harry. And Peter, well, he tries to be helpful in his own way, like getting Harry his favorite toys. Their interactions are full of warmth and a sense of family.
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.
The Marauders would likely have a very negative reaction to the Dursleys. Sirius, having run away from his own awful family, would not tolerate the Dursleys' treatment of Harry. He'd probably confront them right away.
They might interact with suspicion at first. The Black family may be wary of the Marauders' Gryffindor ways and their more liberal views. Sirius would be the link between the two groups. He could try to ease the tension, but it might not be easy. Some of the Marauders like Remus might be more reserved in the interaction, while James and Sirius could be more confrontational if the Black family members start spouting pure - blood nonsense.
One popular one could be 'The Marauders' Era: Harry's Guardians'. It shows how the Marauders would have taken care of baby Harry. Another might be 'Baby Harry and the Marauders' Adventures' which often has them going on various magical escapades while protecting Harry. And there's also 'Marauders' Bond with Baby Harry' that focuses on the special connection they form with him.
One possible plot could be that the Marauders, in some magical time - travel or alternate reality scenario, get to meet baby Harry. They might be overjoyed to see him and immediately vow to protect him. Sirius could be especially attached, given his future relationship with Harry. They might also play with baby Harry, making him giggle with their magic tricks.
The main characters are clearly the Marauders - James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew. And of course, baby Harry Potter. The Marauders are important as they were Harry's father's friends, and their presence around baby Harry would create an interesting dynamic.
Obviously, he would interact with his father, James Potter. They could form a bond that Harry never had the chance to have in his own time. He would also meet Sirius Black, Remus Lupin, and Peter Pettigrew, the other Marauders.
Obviously, Hagrid could be one. He would be so amazed and probably take baby Harry on little adventures around the Hogwarts grounds, listening intently to what Harry has to say.
Often, Dumbledore acts as the wise mentor, guiding Harry. McGonagall is like the strict but fair teacher. For example, in some fanfics, Dumbledore might give Harry a special task related to the greater good of the wizarding world, and McGonagall would help Harry with the practical aspects, like getting access to certain areas of Hogwarts for his mission.
As the story progresses, they start to collaborate. They use their unique skills to help each other out. Harry might use his magic to protect Rogue from certain threats, while Rogue could use her combat skills and mutant powers to assist Harry in battles. A lot of fanfictions explore how they learn from each other's strengths and weaknesses, like Harry learning about Rogue's experiences with being an outcast which he can relate to as a wizard in a Muggle - dominated world.