Alex Edgbert – An 12 (nearly 13)-year-old second-year Gryffindor, who has blue eyes and messy and untamed dirty blonde hair. Alex is the brains of the group and often logic and deductive reasoning to point out clues to their mysteries.
Harry Potter – A 12-year-old second-year Gryffindor, who has messy hair and glasses who became famous within the wizard community by surviving the curse of a powerful wizard. Harry frequently finds himself entangled in dangerous adventures but he always lives to tell the tale. Harry's character represents good intentions, innocence, and the fantasies of childhood.
Ron Weasley – Harry's best friend and Alex's godbrother and second cousin. He is tall, red-haired, and from a respected-but-poor family. He is loyal to Alex and Harry and belligerent to their enemy and Alex's first cousin, Malfoy. Ron uses experience and a process of trial-and-error to solve most mysteries. Ron's character is often overshadowed by Alex's and Harry's, but Ron always manages to succeed.
Hermione Granger – Alex and Harry's other best friend who is clever and well-read. Like Alex, she is intelligent and uses books to point out the clues. Her insight leads her to discover that the monster within the Chamber is a basilisk (along with Alex) and that Polyjuice Potion will allow the group of friends to spy on Malfoy. Principled and fond of rules, Hermione unwillingly follows the boys on their illicit adventures in order to unravel the mystery of the Chamber of Secrets.
Kate Nguyen – Alex's crush and best friend, who often supports Alex's claims. She is the second girl in the group and a half-blood.
Matt Crocitto – Alex's best friend and the newest member of the group.
Tom Marvolo Riddle / Lord Voldemort – A student of Hogwarts from 50 years ago, who opened the Chamber of Secrets. Voldemort became the most powerful and most feared dark wizard of all time and killed Harry's parents and tried to kill him as a baby, but his curse rebounded and rendered him powerless. He is an embodiment of pure evil.
Rubeus Hagrid – The Gamekeeper of Hogwarts and a good friend of Alex and Harry's group. Hagrid is a half-giant and a hairy man with a strong accent and a great liking for dangerous creatures. Tom frames him as opening the Chamber of Secrets.
Albus Dumbledore – The Headmaster of Hogwarts who is strong and wise. He is the only man Voldemort has ever feared.
Draco Malfoy – Alex's first cousin who is slimy and snobby and hates non-pure-bloods.
Percy Weasley – Ron's older brother and a sixth-year Gryffindor who is a Perfect.
Fred and George Weasley – Ron's older brothers and twin pranksters and Beaters for the Gryffindor Quidditch team.
Molly Weasley – Ron's strict and stern, but caring mother, who dotes on Alex and Harry.
Arthur Weasley – Ron's Muggle-obsessed father who works in the Misuse of Muggle Artifacts Office, who has a rivalry with Lucius.
Vernon Dursley – Harry's nasty-tempered Muggle uncle who hates magic.
Petunia Dursley – Vernon's wife and Dudley's mother who is unpleasant and Harry's aunt.
Dudley Dursley – Harry's spoiled, overweight cousin.
Minerva McGonagall – Alex and Harry's Head of House and the Transfiguration professor at Hogwarts, who is strict and stern, but is obsessed with Gryffindor winning Quidditch.
Severus Snape – The Head of Slytherin House and Potions professor at Hogwarts, who gets great pleasure from getting Harry in trouble, but favors Alex as his godfather and honorary uncle. Alex trusts him more than most of the teachers at Hogwarts, with the exception of Dumbledore and McGonagall.
Gilderoy Lockhart – A famous wizard who claims outrageous accomplishments, which Alex does not agree with and the new Defense Against the Dark Arts professor at Hogwarts.
Pomona Sprout – The Head of Hufflepuff House and Herbology professor at Hogwarts, who is kind-hearted and fair to her students.
Argus Filch – The grumpy Hogwarts caretaker, who gets sadistic pleasure in torturing his students.
Mrs. Norris – Filch's beloved cat, who is petrified by the Basilisk.
Cornelius Fudge – The Minister of Magic, who visits to remove Hagrid. Alex calls Fudge an "idiot".
Oliver Wood – A sixth-year Gryffindor and the Keeper and Captain of the Gryffindor Quidditch team.
Colin Creevey – A first-year Gryffindor who idolizes Harry and takes photographs of him. He is the first human to be petrified.
Neville Longbottom – A second-year Gryffindor and a clumsy boy who is Alex's godbrother, friend, and second cousin.
Crabbe and Goyle – Malfoy's dimwitted goons who Harry and Ron impersonate using Polyjuice Potion.
Blaise Zabini – A Slytherin second-year who Alex impersonates using Polyjuice Potion.
Theodore Nott – A Slytherin second-year who Matt impersonates using Polyjuice Potion.
Pansy Parkinson – A Slytherin second-year who is smitten with Draco, who Kate impersonates using Polyjuice Potion.
Millicent Bulstrode – A Slytherin girl who Hermione duels during the Dueling Club.
Justin Finch-Fletchley – A Muggle-born Hufflepuff second-year who is petrified by the Basilisk.
Penelope Clearwater – A Ravenclaw perfect who is petrified by the Basilisk and Percy's secret girlfriend.
Dobby – The Malfoy's house-elf who tries dangerous measures to keep Alex and Harry from Hogwarts.
Ernie Macmillan – A Hufflepuff who believes Harry is a dark wizard.
Nearly Headless Nick – The ghost of Gryffindor Tower who hosts a deathday party on Halloween.
Moaning Myrtle – A ghost who haunts the girls' bathroom, who was killed by the Basilisk 50 years prior.
Peeves – A poltergeist that causes harmless trouble at Hogwarts.
Bandit – Alex's pet pit bull who doesn't trust Scabbers and barks at him.
Hedwig – Harry's pet owl.
Aragog – An Acromantula who Hagrid raises in captivity in the Forbidden Forest.
Fang – Hagrid's friendly dog who is cowardice.