James Potter, as one might guess, is Harry Potter's father. Like Harry, James has perpetually messy dark hair and is roughly the same height by both of their fifth years, though it is never specified if James had been wearing glasses for as long as Harry. He and his wife Lily died a year and three months after Harry's birth (October 31st, 1981) when their hiding place was betrayed to Lord Voldemort, who killed them with the Avada Kedavra curse. He was in Gryffindor House at Hogwarts. With friends Sirius Black, Remus Lupin, and Peter Pettigrew, he is responsible for the creation of the Marauder's Map.
In the books, he is described by a heavy contrast of several different characteristics. During his Hogwarts years, he is at once revered as the best friend a boy could have (Sirius Black and Remus Lupin ), as well as an arrogant, cocksure toerag (Severus Snape and Lily Evans.) Following Hogwarts, he apparently matured, and according to several characters became a kind man, though never did have anything nice to say about Snape. His ability as a master prankster was self-evident as several teachers remember James with more than a little chagrin.
In regard to any Gryffindor in general, James was seen as simply amicable. His relationship with Lily Evans was apparently rather cute, especially to those who were watching it. Though it is never explained quite as to why, his entire relationship with Severus Snape was poor from day one, inspiring several stories which view their poor relations as sexual tension rather than dislike and even outright hatred.
In Badfic Edit
In regards to much of fanfiction, James is rarely if ever seen, and is only hinted at, much the same as J.K. Rowling did in the original septology. In badfic, James can take any terrible role the story gives to him. In many instances, he can even be seen as either a rapist (such as in the Legendary badfic Subjugation,) or seen in highly... sexual relationships with just about every male character in Potterverse, including his own son.
Because the reader learns so little about his actual personality (all accounts of him are secondhand), it may be especially difficult to distinguish an off-color James from a canonically accurate James. Like all characters with ambiguous characterization, it is difficult to tell what is OOC for him and what is not. Therefore, it is recommended to treat any fanfic appearance of James in a speaking or focal role with care and caution, as canon never fully featured him as anything but posthumous. When put near Severus Snape, James Potter in a badfic will almost always try to have…relations of a particular nature. Given that it is indicated rather heavily how Snape and James absolutely despised each other in their Hogwarts years, it is fitting that many stories that invoke this pairing end up being badfic.