Great Basilisk

The Great Basilisk is a female basilisk living in Hogwarts as of 1993.

Youth
The Great Basilisk was hatched in the 11th century by Salazar Slytherin. Slytherin saw her more as a weapon than a person, and he placed an enchantment on her that bound her to obey Slytherin's orders as well as his Heirs'. After teaching her of her "mission" to one day help Slytherin's Heir purge the school of Muggle-Borns, Slytherin left the Basilisk imprisoned in a secret chamber beneath he castle.

18th century
After slumbering for many centuries, the trapped Basilisk was briefly awakened by Corvinus Gaunt, who altered the entrance to the Chamber of Secrets but did not dare to release her, much to her displeasure.

1943
In 1943, Tom Riddle located the hidden entrance of the Chamber and released the Great Basilisk, fulfilling Slytherin's will by ordering her to Petrify three muggle-born students. Satisfied with his success, Riddle tried to move up and ordered her to actually kill a muggle-born girl, Myrtle Warren, a deed which the Great Basilisk was forced to carry out. He used the death of Myrtle to create his first Horcrux, and explicitly ordered the Basilisk never to discuss his Horcrux with anyone.

Meanwhile, the murder proved to be the last straw for the Ministry of Magic, which threatened to close down Hogwarts. Terrified, Riddle sealed the Basilisk back into the Chamber and arranged to make it seem as though the "monster" had been captured by framing a young Aragog for the deed.

1991-1992
On Christmas of 1991, Hermione Granger chanced upon the entrance to the Chamber. She befriended the Great Basilisk, also introducing her to her friends Harry and Ron. Hermione taught the Basilisk more about the outside world, making her realize just how evil and despicable her former masters had been. She suggested the Basilisk accompany them to classes by moving through the pipes inside the thick walls of Hogwarts.

Riddle, then possessing the body of Professor Quirrell, overheard one of Hermione and the Basilisk's Parseltongue conversations. The Basilisk was able to hide Hermione's existence from him and thus she claimed she had been roaming the pipes on her own volition. Riddle angrily forbid her from "taking any other such initiatives". Hermioen managed to work around this order by repeatedly "ordering" the Basilisk to be her friend, meaning the Basilisk was no longer technically "taking an initiative".

Towards the end of the year, Hermione uncovered Quirrell and Riddle's intentions to steal Nicolas Flamel's Philosopher's Stone from the Third Floor Corridor. She and the Basilisk entered the Corridor together and curbstomped most of the challenges within. In the last room, the Basilisk was the first one to have a look inside, and her Gaze rebounding on the Mirror of Erised petrified Quirinus Quirrell and his master Lord Voldemort.

Summer 1992
The Great Basilisk spent July and August waiting for the return of her new human friends. Unable to go back to hibernating, she also cleaned up the Chamber of Secrets.

1992-1993
Shortly after school resumed in September, Hermione and her friends returned to the Chamber of Secrets, once again intent on enlisting the Great Basilisk's help to beat the new challenges in the Third Floor Corridor. Her venom was used to melt a seal of Queenstinger Wax, but it became apparent she would be more of a hindrance than a help for the other rooms, as several were home to live guardians whom Hermione didn't want to make the Basilisk murder outright, which was the only way she could have helped.

Tom Riddle's order not to talk to anyone who demonstrated knowledge of his Horcruxes finally proved problematic when Hermione showed knowledge of Tom Riddle's Diary and the entity within. However, they soon discovered that like the Bond, this command was easily worked around by twisting the meaning of its wording; from then on the Basilisk and Hermione referred to each other in the third person, so that in theory they were never talking to each other, but rather just happened to be speaking to themselves whilst in the same location. Meanwhile, Ron Weasley and Harry Potter "denied" being aware of the Diary's existence. In the end the conversation proved rather beneficial, as, through metaphors and periphrases, the Basilisk was able to convey the Diary's nature as a Horcrux to her friends, though they didn't think much of it, merely deeming it an interesting tidbit to keep in mind.

On Spider Day, Hermione, injured by the Acromantula Nerchog, managed to warn the Great Basilisk of the Acromantulas attacking Hogwarts. Hermione instructed her to come and help, advising her to get Harry to explain to the other wizards that the Basilisk was on their side. (The latter precaution was in the end unnecessary, thanks to Professor Dumbledore's knowledge of Parseltongue.) The Basilisk was decisive in winning the battle against the Acromantulas, especially as she was the one to take down Marchog's mount Norberta.

Following her existence being revealed during Spider Day, the Basilisk was visited by hundreds of curious students in the Chamber, who wanted to meet her or simply see her for themselves. Harry Potter, Hermione Granger and Ron Weasley took turns translating the visitors' questions to Parseltongue and the Basilisk's answers back to English.

On Christmas Day, Albus Dumbledore sent the House-Elf Toddy to give the Basilisk a gift on his behalf. The gift in question was a pair of enchanted goggles which would allow her to keep her eyes open in public while blocking her deadly Gaze. Following this uplifting present, Hermione escorted her to the Great Hall on New Year's Eve, finally allowing her to see it for herself.

Personality
The Great Basilisk is rather ignorant when it comes to the outside world, having been locked up in the Chamber of Secrets for most of her life. She is eager to learn new things and meet new people, forming a strong friendship with Hermione Granger, Ron Weasley and Harry Potter. Feeling guilty about having been the instrument of Myrtle Warren's death on Tom Riddle's order, the Basilisk also seeks to "redeem herself", becoming a loyal defender for Hogwarts.