Peter is remembered as the least intelligent, least attractive of the Marauders. He "tagged along" with James, Sirius, and Remus, but wasn't exactly on the same playing field. Nonetheless, he was their friend, and accompanied them in their pranks, in creating the Marauder's Map, and, with help, in becoming an Animagus. The last is no mean feat, so Peter is not to be underestimated. What he may lack in raw ability, he makes up for in determination.
In Harry Potter's time, Peter is revealed to be the traitor who gave away James and Lily to Lord Voldemort. He had been hiding out with the Weasley family for years in his Animagus form, a rat the family knew as Scabbers. In an act of supreme mercy, Harry spares his life in favor of letting the proper authorities decide Peter's fate. However, Peter escapes and helps to bring about the second rise of Voldemort and acts as his right-hand henchman from that point on. He lives in terror of Voldemort and can be seen as a pathetic, ruined creature.
In Badfic Edit
Badfic writers tend to forget that Peter was a Marauder or, if they remember him, make him ridiculously stupid and sycophantic. (The latter was definitely true, but he was most definitely not stupid. Cowardly, yes, but not stupid.)