Redpills is a term used to describe humans that have been liberated from the Matrix when they have chosen the red pill. Under the truce, select numbers of humans were able to be chosen without unwanted the interference from the Machine Agents. After the truce was rejecteted when the Machines found out about the new and improved Zion, the Architect declared that no more redpills were allowed to be liberated. There are some humans, the Cypherites, that regret taking the blue pill and work against the Zionites.
Most of the playable characters were redpills. Other redpills, like Morpheus or Niobe were non-playable characters.
It is important to note that the natural born Zionites (who cannot jack into the matrix) are not categorised as such.
Bluepills are the opposite, usually describing people who are still trapped into the Matrix or have not given the choice yet.