Sirius Black (Padfoot) ~ James Potter's best friend and accomplice in crime. . . . Also exceptionally talented at magic, he and James helped Peter become an Animagus so they could accompany Remus on their monthly adventures. The . . . incident he is most famous for involved the certain Severus Snape mentioned above. Sirius thought it would be amusing to lure Snape past the Whomping Willow to the Shrieking Shack, where the Formidable Four's nemesis would meet a convenient end in the jaws of a werewolf. Although James foiled the plan at the last minute, its repercussions still are felt, especially in Snape's behaviour.
     Sirius was framed for the betrayal of the Potters, and the murder of Peter Pettigrew and twelve Muggles. He spent twelve (long) years in Azkaban for it, building up a healthy vendetta against Peter, who was the one who framed him. Sirius escaped, intent upon getting back at him, and causing terror in the Magic and Muggle communities. He is currently on the run from the Ministry of Magic, accompanied by a fugitive hippogriff by the name of Buckbeak.

Peter Pettigrew (Wormtail)~ the least talented of the four, Peter needed special help from James and Sirius to become an Animagus but finally managed it, becoming a rat. How appropriate, as when he left school, he joined forces with Voldemort in his lifelong drive to make friends with the most powerful for his own benefit. Not knowing this, and on Sirius's innocent recommendation, the Potters made Peter their Secret-Keeper. Peter turned around and gave their location to Voldemort, resulting in their . . . ah . . . destruction. After Voldemort's downfall, he felt that it would be best for him if he got out of the way of the wrath of Voldemort's other supporters. So he blew up a street full of Muggles, and transformed into a rat, making it look like Sirius murdered thirteen people and landing him in Azkaban.
     As a rat, he was adopted as a pet by a wizarding family and renamed . . . Scabbers. Passed form Percy to Ron, he's actually the first of the four we meet. But of course, we don't know that until after we meet the other two surviving members.

Remus Lupin (Moony)~ Bitten by a werewolf when very young, Remus had no hope of a productive life in the outside world - until Dumbledore became headmaster of Hogwarts and let him come. There he found friends (in James, Sirius, and Peter) who didn't shun him for being a werewolf. In fact, he was the one who inspired them to become Animagi behind Dumbledore's back (illegally, too) so they could go on who knows how many illicit adventures.
     Dumbledore gave him another opportunity as an adult by offering him a teaching position, and he came back to Hogwarts to teach Defense Against the Dark Arts. He also gave Harry Defense Against the Dementors lessons, along with a whole LOT of chocolate.