A Soother is an Allomancer who can burn the External Mental Pushing Metal brass.
A Misting or Mistborn burning brass can Soothe (dampen) another person's emotions, like anxiety in a fight to make the person more willing to fight on. With a powerful Soothing it is even possible to extinguish all emotions, but that has to be done with the help of duralumin by a Mistborn.
A Soother can't read the person's mind. It needs a high skill to identify the person's current emotions, else the Soothing might be spotted. A skilled Soother can make the person feel exactly the way as they wish.
A Soother of old times was powerful enough to control koloss and kandra. In the current time a Soother's power is too weak for that. Only a Mistborn can take control of them with a powerful Soothing while burning duralumin at the same time.
The most famous Soother is Breeze.