MAG-ISA — (subversion) Claudita is a red-head but violence is always her last resort.
Oasis from Sluggy Freelance. She tends to alternate between extreme giddiness (usually when Torg acts nice to her) and extreme jealousy and rage (usually when something comes between her and Torg). Justified since these emotions were programmed into her by Dr. Steve. In addition, she's frequently accompanied by fire-related motifs such as The Phoenix. It turns out that "fiery" is literal; she's pyrokinetic.
Surma from Gunnerkrigg Court. Her daughter, Antimony, also has some of this personality, but it's almost completely tempered by her stoicism. Annie has an associated fire motif, since she has a talent for etherically manipulating fire with the blinker stone. And it's eventually revealed Surma and Annie are both descended from a fire elemental.
Reynardine: (To Antimony) Haha! Well, you may have everyone fooled with your stoic facade, but not me! You have a fire in you, child, a fire that belonged to your mother!
Fyre from the The Beast Legion fit's the trope perfectly. Not only does she have read hair but is a fire bending master.
Trawn from Electric Wonderland, the red-headed editor-in-chief of the Nettropolis Free Press, expresses strong passions for news reporting and keeping her friends/employees safe, although she also has a bit of a temper sometimes.
Haley is a bit of a subversion: she suffers from considerable self-loathing and feels it necessary to hide her true feelings much of the time, believing people wouldn't like her if they really knew her.
Dave from Homestuck, sort of - he was frequently assumed to be by the fanbase (possibly even given a nod on one of Hussie's Formsprings), but after the reveal that he and Rose are siblings, the fanbase eventually agreed on blonde. Hussie's current stance on the issue is that Dave's hair color is whatever you want it to be, so he can still be a Fiery Redhead if you want.
SKO and TKO from Elf Blood are normally generally quite calm and reserved, even kindly. Get them in the same room together and watch their tempers flare.
Marron, the eggroll/bondage emporium waitress turned artist, from Voodoo Walrus is an example of this as her introduction involves threatening a co-worker with an iron maiden and then smashing a phone book into the skull of one of the main characters when he gets a little too blatant with his flirtations.
Mac could be seen as another example of this, but it's been over a year since she was last depicted with her natural red hair color.
Mal from Head Trip. She can be calm, rational, and logical, but she is definitely a Heroic Sociopath and a lot more fun when she's being violent to the stupid people.
The titular character of Nimonadefinitely qualifies, despite being a sidekick to Noble Demon Ballister Blackheart, she's certainly more bloodthirsty and energetic than her superior. Being a shapeshifter, her red hair doubles as her Morphic Resonance, giving her forms an overall red tone.
Autumn Pingo from Precocious often displays herself as innocent and sweet towards strangers and adults, but she has been shown to have a short fuse and can lose control of her impulses when she is upset, but also seems to be very passionate about succeeding in competitions.
Guy from Two Guys and Guy. She once said only considers Wayne her friend because she only dislikes him mildly. She had to take happy pills for her anger issues. And she said she hoped Wayne was dead for using the word "literally" wrong among other things.