Yes, he can talk but mainly through his psychic abilities. It’s not because he can’t, he just doesn’t want to.
We first saw Saiki Kusuo talking with his mouth open during the last scenes of The Disastrous Life of Saiki K‘s main season. This was when he finally sealed away his powers after preventing the catastrophic eruption of a volcano. The next instance was during the last episode of the Reawakened season, where he experiences his daily life as a normal person. During the main season, he mainly used his telepathic abilities to “talk” with the people around him.
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Saiki Kusuo’s character mainly revolves around his powers, his unusual appearance, and his enigmatic behavior. He is someone who had a hard time away from the limelight as a child due to his supernatural abilities. In order to blend in, he used his powers to “normalize” the people’s perception of his appearance.
How Saiki Kusuo Communicates
As telepathy is Saiki Kusuo’s main ability, he used this to his advantage when communicating. He sends his thoughts to the minds of his friends, which makes them “hear” him. At the end of the main series, his friends finally caught a glimpse of him talking and mouthing his famous catchphrase “Yare yare” (やれやれ) or “good grief” in English.
During the events of The Disastrous Life of Saiki K: Reawakened is where he properly experienced life without his abilities. He was required to talk and see the world like a normal person. His powerless life didn’t last long. His brother revealed that since his body has evolved to harbor his abilities, it needs to slowly devolve to normal. This resulted in Kusuo’s uncontrollable power outburst. In the end, he decided to embrace his abilities to prevent the danger of a meteor hurtling toward the Earth.
Immunity to Telepathy
With or without powers, there were still some exceptions to Kusuo’s psychic ability that forces him to physically talk. Not that they’re any stronger than Kusuo, it’s just that they’re able to get around Kusuo’s abilities. There are creatures in The Disastrous Life of Saiki K universe that are immune to his psychic abilities, such as bugs and two people in his life.
Exception 1: Nendou Riki
Nendou is the one character that naturally repels Saiki Kusuo’s telepathic abilities. His thoughtless brain naturally cannot contain Kusuo’s thoughts for communication. He lacked the mental capacity to think which makes it hard for Kusuo to predict his actions through his thoughts. Nendou’s brain capacity can be comparable to a bug that creeps out Kusuo.
The instance that solidified this argument was during the final episode of the main season. When Saiki Kusuo finally prevented the catastrophe and spoke, Nendo’s first comment to that was “It’s the first time I heard my pal talk!”
Exception 2: Kusuke Saiki
Unlike Nendou, Saiki’s older brother Kusuke artificially blocks Saiki’s psychic abilities. Kusuke canonically is a “Doctor” who wears white lab coats and is fond of tinkering with machines. Having invented Kusuo’s left and right transmitters, he also developed a “telepath canceller” that blocks Kusuo’s ability to read his mind. He mainly uses this canceller when he talks or is around Kusuo. Which as a result forces Kusuo to physically talk.
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