A peaceful people who refuse to let their dilithium be used for violence, even when threatened by the Terran Empire of the mirror universe.
The Halkans are a deeply pacifist humanoid species whose planet holds rich dilithium deposits. Kirk visited their council to negotiate mining rights, but the Halkans refused to supply any dilithium that might one day be used to take life.
A transporter accident during an ion storm sent Kirk, McCoy, Scott and Uhura into the mirror universe, where the ruthless Terran Empire was ready to annihilate the Halkans for the same refusal. The Halkans' stance remained unchanged in both universes, and the episode Mirror, Mirror made them a lasting symbol of principled nonviolence.