While the Harry Potter franchise started as kids with magical powers trying to find their place in the world, it turned into a massive war. On one side was Lord Voldemort and his Death Eaters, and on the other was the members of the Order of the Phoenix. As with any faction in a war, both sides had characters that straddled the line between heroes and villains.
This was true with the warriors who were part of the Order of the Phoenix. While their overall goal was to bring the reign of terror started by Voldemort to an end, that does not mean that everyone in the group was perfect. Some of the members were just one action away from drifting completely to the dark side.
Sirius Black
The most heroic of the Order of the Phoenix members is also the man that most people in the Harry Potter world assumed was the greatest villain. Sirius Black was Harry's godfather, but he was also imprisoned as Azkaban for crimes he never committed. When Sirius escaped, he immediately set out to protect Harry and took an active role in battling Voldemort again. His ultimate sacrifice to save Harry proved he was the greatest hero of them all.
Minerva McGonagall
Minerva McGonagall was the most responsible teacher at Hogwarts, and when the time came to go into battle, she was also one of the fiercest warriors. Professor McGonagall was not a part of the Order of the Phoenix in the First Wizarding War. She joined up when the new resistance movement began before the Second Wizarding War shown in the Harry Potter series. She not only survived, but she led the resistance when Voldemort attacked Hogwarts and saved countless lives.
The Weasley Family
When the Harry Potter franchise began, Ron Weasley was the face of the Weasley family, and he appeared to be a bumbling, awkward child, who never seemed like he came from warrior stock. That is far from the truth. Ron came into his own, which is no surprise as his family was part of the Order of the Phoenix, with his dad Arthur and mother Molly being strong warriors. They were also good people, taking in Harry and others and proving to be virtuous through the Second Wizarding War.
Albus Dumbledore
Albus Dumbledore is the man that most people would put at the top of the list of most heroic members of the Order of the Phoenix. However, there are reasons to see him in a lesser slant. He is a man who did what was needed to win the wars, from fighting Grindelwald to beating Voldemort twice.
However, his methods were not always pure. Dumbledore used children as pawns in his war and was responsible for the death of more than one child put into his care. Dumbledore was a great man, but his actions were not always that of a pure hero.
Nymphadora Tonks
Nymphadora Tonks was a British half-blood witch who initially joined the Ministry of Magic to become an Auror, working for Alastor Moody. She ended up joining the Order of the Phoenix while working undercover for the Ministry. As a spy, she helped the Order and was then assigned to Hogsmeade to protect it from Voldemort. She ended up falling in love with and marrying Remus Lupin. Tonks ended up dying in the Battle of Hogwarts, murdered by her own aunt, the Death Eater Bellatrix Lestrange.
Remus Lupin
Remus Lupin was part of both the original Order of the Phoenix and the resistance version years later. He was, in his younger years, best friends with Sirius Black, James Potter, and Peter Pettigrew. However, he also held a dark secret. He was a werewolf, bitten as a child by Fenrir Greyback, and therefore could never be fully trusted when the moon was full. Remus was a teacher at Hogwarts until his secret was revealed, and he was forced to resign from his position. He died in the Battle of Hogwarts, alongside his wife, Tonks.
Kingsley Shacklebolt
Not much is known about Kingsley Shacklebolt from the Harry Potter movies or book series. However, he was given a detailed history in Harry Potter: Wizards Unite. Shacklebolt was a British full-blood wizard who worked for the Ministry of Magic.
Shacklebolt worked as a spy from within the Ministry and helped keep Sirius Black safe after his escape. He fought in the Battle of the Department of Mysteries and was assigned to protect the Muggle Prime Minister. He ran away when Voldemort returned and finally returned for the Battle of Hogwarts, where he became the new Minister of Magic after the war ended.
Alastor Moody
Alastor Moody was a heroic figure in the Order of the Phoenix, but he suffered from one mishap that damaged his reputation. The man known as Mad-Eye Moody was captured by Barty Crouch Jr., who locked him away and posed as Moody when he became the Defense Against the Dark Arts professor at Hogwarts, setting up Harry Potter for a fall. Moody finally returned and re-joined the Order of the Phoenix, but he died in the Battle of the Seven Potters, protecting Harry.
Severus Snape
Many fans consider Severus Snape to be one of the greatest of the Order of the Phoenix because of what he did at the end of his life. He spent most of his time in the Harry Potter franchise as one of Harry's most despicable antagonists. It turned out he was a double-agent and was working for the Order of the Phoenix the entire time, but that does not excuse Snape's treatment of Harry and his willingness to turn many young wizards into possible Death Eaters through his vindictive and dangerous teaching.
Peter Pettigrew
The most dastardly and villainous member of the Order of the Phoenix was Peter Pettigrew. He was part of the Order during the First Wizarding War, but he ended up as a double-agent. Unlike Snape, who spied on the Death Eaters, Pettigrew was sent to spy on the Order for Voldemort. He was best friends with Sirius Black, James Potter, and Remis Lupin and helped create the Marauder's Map. However, he betrayed James and Lily and was responsible for their deaths, framing Sirius for the betrayal.