Stephanie mentions Finn's hot temper right off the bat, right before we get to see his response to Timothy trying to pick a fight with him. He flash steps right up to Timothy, delivers a quick Badass Boast that doubles as a death threat, calls the grown man an insolent child and then tells the guy to get out of his sight.
In the Branion series by Fiona Patton, the royal family of an alternate Britain is literally this. The sovereigns are the Jesus-style avatars of a fire god, and they commonly have red hair. They also have tempers, which is not surprising given that effectively having a fire god inside you is not good for your mental stability. Justified by the presence in their family tree of Danes, Welsh and Irish.
The Ashtown Burials series, by N.D. Wilson, has the reddish-brown haired Diana Boone. She doesn't have a particularly quick temper, but she is a skilled warrior and ace pilot, and you don't want to be on the wrong side of her when she does get angry.
As is her son, Ben Skywalker. The striking similarities between the two (particularly their personalities) are brought up fairly often in profic, especially in Sacrifice, and a LOT in fanfic. To be honest, probably the only notable physical feature he inherited from Luke are his blue eyes.
Ygritte, a wildling, from A Song of Ice and Fire. All redheads are believed to be "kissed by fire" in the wildling culture and have a reputation for spirit. Averted in Westeros proper, where red hair is associated with the more genteel Tullys. Rickon Stark, however, inherited the Tully red hair and is the most expressive of his siblings, although this may be due to his youth.
Ginny Weasley, as well as Molly and the rest of the Weasleys. A more mellow example is Percy, and he's regarded as a stick-in-the-mud.
Harry's mother, Lily, was also like this, mostly around James before they ended up hooking up. Hmm, maybe the Potter men have a thing for fiery redheads.
Ciaphas CainHERO OF THE IMPERIUM's comrades Colonel Kasteen and Trooper (later Corporal) Magot fit this trope to a T, Magot almost to the point of being a Psycho Lesbian. And, no, the two redheads are not a couple.
Justified with Anne Shirley whose temper was at its fieriest when she was teased about her red hair (being one of the few characters whose fire is connected to their coloring). She chewed out the neighborhood busybody and also responded to being called "Carrots" by breaking her slate over a schoolmate's head. Nobody ever teased her about her red hair again, after the social snubbing Gilbert got for five or so years. Ironically, they end up marrying. note It's ironic for Anne. That was Gilbert's goal since the beginning.
Anne never quite gets over her dislike of her red hair. She didn't even like it when her children inherited it. When Jem is born and the nurse tells her his hair will be red, Gilbert reports to Marilla that "Anne is furious with her, and I'm tickled to death!"
Aviendha (and, to a lesser extent, all Aiel women) from the Wheel of Time series behave like this. They are quite proficient with the spears and knives they threaten men who make unwanted advances with.
In Robert Graves' King Jesus, Jesus has red hair, that being one of the eight "signs of royalty".
Herrenna the Henna-Haired Harridan, a side character in The Light Fantastic has red hair (per her title) and absolutely no patience or pity for either Rincewind or Twoflower. Oh, and she has a very big sword.
Alanna of Trebond of Song of the Lioness (the first book in the Tortall Universe) has bright red hair and a temper to match. Her squire later remarks how she's known for her sharp edges—sword, knife, and tongue.
Ditto with Tris in Tammy's Circle of Magic series. She's very prickly from years of verbal abuse and abandonment. She does eventually warm up to her new friends, but she's still quick to sarcasm and doesn't tolerate fools gladly.
Ce'Nedra from David Eddings's Belgariad. Not just her husband Belgarion, not just their royal court, the entire kingdom of Riva is focused on keeping Queen Ce'Nedra happy. Because the alternative is just too grisly to contemplate.
Maedhros, Amrod, and Amras from J. R. R. Tolkien's The Silmarillion are the only elves to be described as redheads who fit the personality type. They're fierce, rash, more violent than is really healthy, and not particularly wise or patient... but then so are all their darker-haired siblings. A lot of fans assume their mother Nerdanel's also redheaded, thought the author only ever said her father is, and both of their personalities are the complete opposite: calm, soft-spoke, patient, and wise. They also had the sense not to join the rebellion of the Noldor, which is why they're still alive.
Karen in The Three Worlds Cycle. Specifically in the first quartet, where she appears as a main character. She takes on Rulke, for Io's sake, and harasses every main character through the whole quartet. And she just happens to be described as having "extremely fiery red hair"...
Sarah in Tales of an Mazing Girlis pretty firey in the right circumstances, if she often keeps her sass to herself.
Jennsen Rahl in the Sword of Truth series. In-story, red hair is often (falsely) stereotyped as the hair color of witches among the blond-haired D'Haran purebloods, rather than indicative of personality.
Margot Maynard in Elinor M Brent-Dyer's Chalet School series. Her hair is constantly described as 'reddish-gold' and she has an absolutely rotten temper, which she blames on her 'demon'. She tries to control it, but it often gets the better of her, culminating in her nearly killing a girl with a bookend in The Chalet School Triplets. Her cousin Sybil Russell also qualifies, at least as a child, until an accident with a kettle and her younger sister has a drastic effect on her personality.
I, Robot has engineer Mike Donovan, described as possessed of perpetually unkempt red hair, a short temper, and a generally Hot-Blooded personality, in contrast to his calmer dark-haired work partner Gregory Powell.
Princess Elizabeth of The Royal Diaries' Elizabeth I: Red Rose of the House of Tudor can swear up a storm when she wants to.
The main character of the Dear America book A Coal Miner's Bride: The Diary of Anetka Kaminska complains about everything about her looks except her red hair. At one point, she suddenly remembered that she was her mother's fiery redhead and started yelling at her ungrateful husband with a list of the things she does for him.
'Red' Kelly and his sister Kelly Kelly in the Young Bond novels. In fact, the whole Kelly clan might qualify.
'Red' Blaze from the Civil War and Floating Outfit novels of J.T. Edson. Blaze is the most hotheaded and impetuous of Edson's heroes.
In Death series: Feeney has ginger hair. He may seem rather calm, but he can be explosive when he's angry.
In Asimov's Lucky Starr series, Lucky's sidekick John Bigman Jones has pale red hair, a short temper, and a generally emotional nature. Lucky, on the other hand, is calm and brown-haired.
In Tom Robbins' Still Life With Woodpecker, in addition to the two red-haired protagonists (Fallen Princess Leigh Cherry and Outlaw Bernard Mickey Wrangle), there's a involved sideplot/conspiracy theory around the extraterrestrial origins of redheads.
The princesses of A Brother's Price are all noted for having red hair, referred to as "the royal red" at a few points. Each princess has a different personality and none are exactly mercurial, but strength and passion are part of all of them.
Cord MaKiy from the ColSec Trilogy, although it's somewhat mitigated by his introversion.
Jessie in High Society by Ben Elton. Her hair colour (dark auburn/reddish-brown) is referred to only once, but combined with her personality she is an obvious example of this trope.
Kaylana the druid in Villains by Necessity. Living up to the trope by answering any attempts to approach her with a staff knocked over the poor man's head.
Simona Ahrnstedt gives us Beatrice Löwenström in her debut novel ÷verenskommelser. She's intelligent, competent and assertive. But alas, many people in this era (the 1880s) prefer quiet and subservient women. And being at the mercy of her abusive uncle, who bullies her into a marriage with a man, who treats women like dirt under his shoes, she finds that out the hard way. But she's not afraid to call her oppressors out on what creeps they are, even if that only makes them even angrier with her...
Averted with Beata Jensdotter in "De skandalösa". Despite being a redhead, she's less fiery than the brunette protagonist Magdalena.
Midnight's Children has Saleem's sister the Brass Monkey, who got her nickname because she's one of these. She is also literally fiery, in that she lights shoes on fire to get attention. Her hair darkens to brown when she turns nine, and she subsequently mellows out a bit.