Vjylku

So in a webtoon that I am drawing/writing everyone is born with 1 or 2 psychic abilities that can range anywhere from teleportation, sensory manipulation, thermal conductivity, elemental kinesis abilities, etc.

And everyone’s ability is explained via science, some of it real and some of it exaggerated but still mostly accurate, for example an electrokinetesis in my story generates the electricity they control by amplifying the electrical current that is constantly produced from our cells and then discharging it.

Heat works the same way as our cells naturally produce heat and I could have fire users also amplify the heat in their body to generate flames but fire requires a fuel source to burn and I can’t think of any way that a person might be able to use fire without changing how human biology would work in my story, unless I make it so that fire users need to carry around a fuel source of some kind which could be cool in some situations but put them at a massively unbalanced disadvantage in most situations.

Pyroflatulence

Well you did ask. And it is a real thing.

Pyroflatulence or flatus ignition, is the practice of igniting the
gases produced by human flatulence, often producing a flame of a blue
hue, hence the act being known colloquially as a "blue angel", "blue
dart", or in Australia, a "blue flame". The fact that flatus is
flammable, and the actual combustion of it through this practice,
gives rise to much humorous derivation. Other colors of flame such as
orange and yellow are possible with the color dependent on the mixture
of gases formed in the colon.
https://en.wikipedia.org/wiki/Fart_lighting

Depending on the individual's gut bacteria, farts can contain large quantities of hydrogen and methane, both of which are highly flammable.

You can of course find many examples of this on YouTube.

How can this be activated?

We know that in times of fear or stress, people are much more likely to fart. This would make the weapon available at exactly the right moment. Just carry a pocket lighter. Perhaps there is some evolutionary adaptation that causes a spark to light the fart.

Edit

Inspired by @Agrajag to explain the spark generating method. I suggest that these people are also contortionists and can create the spark by gnashing their teeth together. They can be identified by their lack of eyebrows.

A bombardier beetle creates a toxic spray by mixing two fluids into a single orifice. The fluids are chemically relatively inert when kept separated but produce a lot of heat when mixed. With only a bit of imagination, you could have a human producing a similar pair of chemicals that spontaneously ignite when mixed. Highly modified salivary ducts and a somewhat fire-proof mouth lining could then let them spit fire.

But I can't imagine it would be terribly efficient. Maybe good enough to light a cigarette, or give someone a fright, but hardly suitable for incinerating enemies.

One issue would be volume of fluid. A salivary duct produces only fractions of a ml per second, so in order to produce a significant volume of 'fire' the fire-starter would need some sort of internal bladders to store the stuff until they were ready to spit. Maybe mammary ducts would be more appropriate in that case...

It could be explained by Ketosis. Ketosis is a state that your body enters when you deprive yourself of carbs (less than 20g a day is optimum), the body then begins to burn fat to produce ketones.

Since burnt fat exits the body through the lungs, your character could exhale large amounts of the fat that could be ignited through a means of spark or a ignition point. A character from the Tv series 'Grimm' called 'Daemonfeur' uses this method (pretty sure).