Harry Potter himself is of course very common. He's usually the central figure in these fanfictions. Hermione Granger is also often included as she is Harry's friend and a brilliant witch. Ron Weasley too, as he's part of the trio. Sirius Black might also be a common character since he has a connection to Azkaban.
Sirius Black is often very popular. Since he's been freed from Azkaban, he brings a lot of mystery and a connection to Harry's past. Harry himself remains a central figure, of course. His growth and development after the events of Azkaban are always interesting to explore. Hermione is also popular as she continues to be the brains of the group, coming up with clever plans and solutions.
Harry Potter himself is of course very memorable in this fanfiction. His growth and development as the 'King of Azkaban' are central to the story. He might display new traits like increased determination and a darker side due to the harsh environment of Azkaban.
It could be Harry Potter himself, of course. He might be on a mission to either free someone wrongly imprisoned in Caer Azkaban or to stop a new dark threat emerging from there. Hermione could also be a main character, using her intelligence to figure out the mysteries of Caer Azkaban.
One popular original character is often a new student who has some special ability related to divination. This character can add new elements to the story, like predicting things about Sirius Black that no one else can.
Some fanfics introduce a new character, a seer who can see not just the future but also alternate timelines. This character is unique as they can provide insights that no one else can, and often their visions drive the plot forward in unexpected ways.
Obviously, the child is one of the main characters. There might also be some prison guards who play important roles, either as antagonists or those who gradually change their views towards the child. And perhaps some other inmates who interact with the child could also be considered main characters.
There could also be new characters introduced specifically for this fanfiction. Maybe there's a vampire hunter who comes to Hogwarts thinking the vampire is a threat. This character would add a new layer of conflict and intrigue. Additionally, the teachers at Hogwarts, like Dumbledore, would still play significant roles as they try to maintain order in the school while dealing with this new and rather unusual situation.
Another popular original character could be a mysterious professor who has a hidden agenda. This professor might seem friendly at first but is actually working for a secret organization. They add an element of mystery and suspense to the story. They could be secretly studying the secrets of the Azkaban prison or have a connection to Sirius Black's past that no one else knows about.
Hermione Granger is also popular. In fanfiction, she could be using her intelligence to come up with new ways to improve the relations between the houses, perhaps by creating study groups that include students from all houses.
Definitely Harry Potter. He's the face of the franchise, so in 'harry potter system lord fanfiction', he's often the center of attention. The system can give him all sorts of new abilities and make his journey even more epic. Ron Weasley is also popular. His loyalty and his growth can be really interesting to explore with the addition of a system. He might become a great 'lord' in his own right, ruling over a territory or group of wizards. And don't forget about Dumbledore. In some fanfictions, he might be involved with the system, or the system could be something he left behind for the main character. His wisdom and power can be amplified in these stories, making him a really engaging character.