Final answer:
The Kelvin scale is unique because it is an absolute temperature scale with 0 K being absolute zero, the theoretically lowest temperature possible, a characteristic not found in Celsius or Fahrenheit scales.
Step-by-step explanation:
One characteristic of the Kelvin scale that is not a characteristic of either the Celsius or Fahrenheit temperature scales is that it is an absolute temperature scale.
This means that 0 K represents the absolute zero, which is theoretically the lowest possible temperature where all molecular motion ceases. Unlike the Celsius and Fahrenheit scales which are relative to the properties of water and an arbitrary definition of temperature points respectively, the Kelvin scale's zero point is based on the fundamental properties of matter itself.