Rishi is defined as rishaa mantra dastara. Rishis are those who saw these vedic mantras, literally saw them. These opanisas o the waders themselves are the spiritual realizations of the ricis. They have been given by god through the ricis down to us piatyand teaching lineage for the benefit of humanity.
Swami Sarvapriyananda teaches the Katha Upanishad, which is one of the most popular Upanishads. The scripture concerns the story of a young man who goes to the house of death to find out what happens after death. The reply describes the nature of reality and the meaning of life.