Saiki Kusuo is an enigma, and not just because of his pink hair and green tint glasses. The main question everyone has is whether Saiki talks to the characters or not. This has caused lots of confusion- does he use telepathy or does he just not speak at all?
The answer to the question is that yes- Saiki speaks, but through telepathy. Owing to his personality, Saiki is more comfortable with telepathic conversations. When you suffer because of your abilities and have an existential crisis, talking with your mouth seems too simple, right?
For the most part, we are all left wondering when Saiki is conversing with the characters, and when we are just hearing his thoughts (fun theory, what if his thoughts are him talking to us via telepathy and breaking the fourth wall?) Despite so, Season 3 episode 2 gives us some clarity on what the truth is.
Nendo mentions that this is the first time he heard Saiki speak. Why? Remember, Nendo is canonically an idiot. Subsequently, Saiki’s telepathy never worked on him due to his inability to think. Therefore, no conversation took between the two of them. Props to Nendo for still being a true friend for Saiki throghout, though!
Furthermore, both Kaido and Teruhashi point out that they’ve been hearing Saiki. This again proves that Saiki uses telepathy to express himself.
In “The Disastrous Life of Saiki K: Reawakened”, we get another scene which proves that Saiki doesn’t talk normally, but still holds conversations.
Kusuo’s elder brother Kusuke wore a device that cancelled Kusuo’s telepathic abilities. However, Kusuke removed it after helping his younger brother seal his powers. The fact that Kusuke asks Kusuo to speak “normally” proves that he does talk, just in a unique, psychic way. Moreover, you would notice that he definitely opens his mouth in shock or surprise way more after sealing his abilities. This subtle change also shows that he just never opens his mouth to speak (lucky, its an ability introverts are jealous of.)
Regardless of anything, we all can agree that Saiki and his medium of conversation is one of the many things that make this anime so special!