The "Whyman" is the name Ryusui gives to the source of powerful radio wave messages the Kingdom of Science receives after the creation of antennae. The name is chosen as the messages consist only the word "why" in Morse code.
The true nature of the Whyman is unknown. Although it seems he led Senku's party in the direction of the petrification device in order to re-petrify the Earth. In Chp 138 a signal takes over the radio in an attempt to activate the device, the distance being Earth's diameter and the time 1 second. It is unknown if the two signals were from the same source, however the voice, as noted by Ukyo is the same voice as Senku. Some in the Kingdom of Science speculate the source is a group of survivors of the Petrification. Others, including Senku, believe they could be the ones responsible for the Petrification.
He is apparently on the moon.
Determining the nature of the Whyman is a major goal of the Source of the Petrification Saga.
- Its possible Rei is the source of the signal using the copy of Senku's voice. As it is the only "being" out in Earth's orbit that could be near the moon.