Saiki has an extremely uncaring personality. As a gifted psychic that can do anything he wants, he has long stopped caring about the world; however, he has shown to be extremely cautious, as he often runs from Nendou, the only person he fears, and is prone to immediately teleporting to far away places upon spotting an insect. Saiki has a sweet tooth, so he loves sweets, especially Coffee Jelly.

Fourteen days after his birth, Kusuo uttered his first word without moving his mouth and 1 month after birth he took his first step in midair. At the age of 1 was the first time Kusuo has voluntarily used his abilities, when his mother said they were out of mirin, Kusuo teleported away and returned with several bottles of mirin.

Saiki is a gifted psychic. Having obtained his powers since birth, the first signs of Saiki's powers came from when he was able to hear the voices of his parents and he subsequently telepathically spoke back to them. Two months later, he demonstrated the ability to walk in the air. Saiki has stated that in three days, he can wipe out humanity. The full extent of all of his abilities is unknown.