Quotes

According to Ava Paige, Joan shares Thomas's  "too changeable, unpredictable" nature. Joan is a very kind-hearted, brave-spirited, natural leader. Many people tell her she is brave, such as friends, family, strangers, and even WCKD Staff. She has the same "brooding" look as her father, which always had her branded as a rebel; her brother has a rebellious streak just like him. Joan is characterized by her sarcastic, smart sense of humor, and laid-back demeanor.

Joan is somewhat short-tempered and thus has trouble controlling her anger, such as when she sees bullying. She can be derisively sarcastic and moody, especially when someone tries to manipulate her or forces her to obey. She does not like to follow rules, as shown when she escapes the Glade to go on the quest to 'save' George, and when she violently slapped Nick (the leader) across the face for betraying her. Like many heroes, Joan is very reckless and tends to do or blurt out things before thinking, causing her to get into a lot of trouble. She feels responsible for everyone and anything which goes wrong.

According to Gally and Minho, Joan is obtuse, meaning she doesn't always see the obvious even when it is right in front of her, such as people's feelings and what they are trying to say indirectly. She has romantic feelings for Newt over the course of the series, but she finally confirms them in The Maze Runner, by accident. Despite this she is perceptive when she needs to be, at the end of the Scorch Trials, Joan could tell that Janson wanted to keep her alive and gambled her life knowing that she was too valuable to Ava Paige, and to WCKD, for her to die at that time.

Although he has many friends in the Glade, Joan is still considered a troublemaker in school with a track record of slip-ups, like when she punched Billy, or when she went into the Maze during the day to help George. Despite being the only girl, she has never been one of the "cool" kids in the Glade because she hangs out with supposedly "un-cool" people, such as new greenies when they arrive, and Chuck. Possibly because of her bad reputation in the Glade, Joan feels a strong level of affection and attachment to her close friends. In the Scorch Trials, Brenda states that Joan had a much better nature than people assume: powerful, but gentle and helpful, a person that would guide ships safely to the shore rather than destroy them.

After getting her memories taken by WCKD, living in the Glade and fighting monsters, Joan has developed a "wolf stare" that says "No matter how bad you think you are, I'm worse," which enables her to scare off people like gangsters. She is not afraid of anything in the world anymore, including cranks. She notes that it is probably Randall Spilker's influence on her, as he essentially raised her from when she was 7 years old. Despite being very confident in her own abilities, Thomas can sense there is a quiet sadness in Joan similar to Newt, like she saw her destiny and knew that one day she would face a monster she couldn't beat.

After the 'Marcus' incident during the Scorch Trials, Joan is starting to show a darker and more dangerous side of herself in her personality. She begins to question herself about her morality, and if she's really a good person or not. She wonders about the people she had previously killed: Barkley, George and even about abandoning Thomas when he was first taken by WCKD when he was 5 years old. After her memories begin return, she tries to make up for her one-sided tattered relationship with Thomas by being more frindly toward him and trying to protect him from any dangers. While battling WCKD at the end of the Scorch Trials, Percy taps into his rage and controls poison to overwhelm Akhlys—to the point where even Annabeth is terrified of him. He was shown to be more prone to aggression when he vows to destroy Gaea and avenge Bob and Damasen, something which doesn't seem to disappear towards the end of the book.

After the events of The Blood of Olympus, Percy seems to be less impulsive. He has grown a bit tired of the constant dangers of a demigod and looks forward to living a normal peaceful life as much as possible. The cunning sorcerer, Setne tried to manipulate Percy, but it backfired on him as Percy succeeded in capturing him. Percy gives more priority to his family and even turns down Apollo's offer to go on a dangerous quest to help the latter regain his godhood. However, he does small favors to others as it is his principle to aid demigods as much as possible, such as escorting Apollo and Meg to Camp Half-Blood at for Meg’s sake and training Magnus Chase for a voyage across the sea at Annabeth’s request.

At the end of The Tower of Nero, Percy seems to be more lighthearted and even softens up to Apollo than before(much better than how it was at the start, in The Hidden Oracle) as he has settled down with the hope of going to college, which is a much different view point from the start as he actually looks forward to it and somehow, with the help of Annabeth has actually done well in his exams. He has lost his impulsiveness and has matured to a lighthearted cheerful person who is at his happiest with Annabeth.

In The Last Olympian, Percy expresses to Hermes a hope that the gods can change for the better, although Hermes is doubtful about this. The gods actions during the Second Giant War, including breaking some of their promises to Percy, seem to prove that Hermes was right. However, in The Sun and the Star: A Nico di Angelo Adventure, Hades reveals that Percy's optimism and attempts to get the gods to change their nature were not without success. According to Hades, Percy had changed a lot of things for the gods, making them rethink their priorities, standards and behaviors. Hades at least changed because of this, rethinking sending his son on a dangerous quest after realizing that Nico could lose someone else that he loved in the process and visiting Nico in a dream with Nico's mother and sister to give his son closure and express his pride in Nico.