Nagini

Nagini is shape-shifting woman born in Indonesia who was afflicted by hereditary Maledictus curse. This condition eventually caused her permanent change into large snake, about 12’ long. In her snake form Nagini met Lord Voldemort and became one of his horcruxes. She is introduced in Goblet of Fire (4th book), and Neville Longbottom kills her with the Sword of Gryffindor at the Battle of Hogwarts in Deathly Hallows.
Her human backstory is revealed in Fantastic Beasts: The Crimes of Grindelwald. 
Voldemort communicates with her through parseltongue.
== Canon ==
=== Fantastic Beasts: The Crimes of Grindelwald ===
Nagini spent her early youth in Indonesia, though she looks more East Asian than Sutheast Asian, implying Indonesia was not her ancestral place, or at least that she has some additional ancestry beside Indonesian. There was curse in her blood causing her to changing into snake form time to time; while initially able to change of her own, it is revealed that curse one day will trap her permanently in animal form. By 1927 she lives in France in magical circus (to which she was forced againts her will) in which she was presented as "freak" and mistreated by circus's owner, Skender.
When fellow outcast named Credence Barebonem had joined circus, Nagini and him got closer and eventually developed romantic feelings to each other. By the time of movie plot, pair plots to escape from circus during spectacle and this plan is ultimately successful.
Nagini goes with Credence on quest to find his biological family and two eventually make their way to gathering of Gellert Grindelwald's followers. Nagini is distrustful towards assembly and points out to Credence that people around Gellert are mostly of Pure-blood and they despise underbeings like her (as she has no magic of her own, shape-shifting being solely result of curse) and people of unknown Blood Status like Credence. Grindelwald and his followers do not respect people like Credece, they want him because of his powers as Obscurial. Ignoring his beloved pleas, Credence decides to join Gellert, as wizard implied he knows where is Barebon's biological family. 
Nagini is unable to follow Credence, as she is separated by blue fire that only allows those loyal to Grindelwald; Nagini opposes Gellert, so she cannot cross.
After being abandoned by man whom she loved, Nagini goes to Hogwart with other enemies of Grindelwald, to seek help of Albus Dumbledore. 
=== Life after Credence and serving Voldemort ===
Fantastic Beasts: Secrets of Dumbledore originally was meant to have Nagini's arc within; howevere COVID-19 pandemic caused shooting of movie to be delayed and when it resumed again, Nagini's actress Claudia Kim was heavily pregnant and unable to attend, causing character of Nagini to be cut from movie.
Though Credence ultimately turned from Grindelwald, it's unknown if he and Nagini ever met again. It possible by the moment Barebone's changed his allegiance, Nagini had already fallen victim of Maledictus curse and was permanently stucked in animal body. 
As during Harry Potter series Nagini never appears in human form, it is presumed (though not confirmed) she is now snake permanently. While during her time with Credence Nagini was just and kind person, who showed disdain towards Grindelwald and his minions for their treatment of magical people who are not pure-blooded, snake Nagini from Harry Potter original series seems to have no problem with serving dark and evil man like Lord Voldemort despite fact he has much more radical views and he is more ruthless and cruel than Grindelwald ever was. 
It is unknown wheter Nagini turned from good and become evil person, radically changing from whom she was in her youth, or if Maledictus curse into its final state simply took away all her human personality and feelings, leaving only animal instinct. 
=== Fandom ===
=== Shipping ===
Credence Barebone/Nagini
Tom Riddle/Nagini 
==Resources==
*Nagini (Harry Potter) works on Archive of Our Own
==References==
Category:Harry Potter Characters
Category:Fantastic Beasts