A red piller is someone who is proficient (or at least thinks that they are) in the act of redpilling. In most cases, the red piller is politically motivated and attempts to lead their target to their side of the
political spectrum through a variety of mediums- most commonly memes, (edgy) jokes, and (mis)quotes.
While there are examples of red pillers all over the
political spectrum, they are most prevalent on the far left and right, along with fringe groups of fascists, communists, or anarchists -recently being used nearly exclusively by the right wing.
The "red pill" term comes from The Matrix, in which a character chose between a blue pill and a red pill, where the blue pill would let them live freely and without knowledge of the inner workings of their world, and
the red pill would allow them to "wake up" from their ignorance, and learn the "truth."