Which particle carries a negative charge?

The electron, discovered in 1897 by J.J. Thomson, who showed that cathode rays were made of particles far lighter than any atom - which meant atoms had parts, and the word atom, meaning uncuttable, was already wrong. He called them corpuscles; the name electron won.

It has no known size. As far as any experiment can tell, it is a point with mass and charge and no measurable extent, which is a genuinely uncomfortable thing for a physical object to be. It also is not orbiting the nucleus like a small planet: the picture everyone is taught is a century out of date, and the modern description is a probability cloud where the electron does not have a definite position between measurements.

Almost everything you experience is electrons rather than matter in any solid sense. Chemistry is electrons rearranging; electricity is them drifting, remarkably slowly, while the signal travels near light speed; light is emitted when they change energy state; and the reason your hand does not pass through a table is electron repulsion and the exclusion principle rather than anything touching. Thomson won a Nobel Prize for showing the electron is a particle, and his son later won one for showing it behaves as a wave. Both were right.