Answer:
1. Sirius Black
Sirius turns out to be Harry's innocent godfather, however before returning to school Harry believes - as does everybody else - that he is a dangerous mass murderer. He also doesn't know before returning to Hogwarts that Sirius is his godfather but he soon discovers this and the belief that Sirius betrayed James and Lily Potter, leading to their death. Harry later gets told the truth by Sirius and Lupin and learns to trust Sirius who is now his only living relative other than the Dursleys.
2. Remus Lupin
Lupin ends up being a werewolf. The whomping willow was planted for his own protection, and the tunnel to the Shrieking Shack was also created for him. This was so that, at the full moon when he was a child, he could go down the tunnel to the Shrieking Shack and stay there until he returned to being human. The whomping willow was planted to keep students away from the tunnel.
3. Peter Pettigrew/Scabbers
Peter Pettigrew turned out to be a rat Animagus, and was living as Scabbers for 12 years with the rest of the world believing that he was dead, an image that he put on to make it look like Sirius was the murderer of the Potters, even though he was the one who sold them to Voldemort, not Sirius. He was friends with James Potter, Sirius Black and Remus Lupin in their school years but was never as close as the other three.