Saiki is Demi NOT Aroace!!!
To be honest, I get it Saiki being aroace is one, if not, the most headcanon. Despite that, these evidence behind it didn't hold it very well among fandom. Let me explain with basic information.
First of all, aroace is abbreviation of Aromantic asexual for someone may or may not feel other forms of attraction that are neither romantic nor sexual such as sensual, platonic, and alterous. A person who are aroace could date or have relationship with others even with their zero attraction.
Starting from the most obvious clue, Saiki shows "little" interest in romance given his attitude toward attraction is very contrast than most characters. And I understand where it comes from, based through some manga panels and anime scenes. Even so, there's a big difference between "Saiki experiences little to no romantic attraction." and "Saiki is completely incapable of ever developing romantic feelings.", the latter is the one we should focusing on. We all knew based on his backstory, that he isolated himself from others around elementary age because of his psychic powers. However, his intial rejection of romance and his difficulty understanding human emotions aren't necessarily proof that he is unable to have feeling.
In fact, he believed he doesn't need people, only to slowly form a genuine connection from his classmates turning into his friends. His denial core is fundamentally tied into his big arc character development; someone who he IS from the beginning after distanced himself from human emotions in his completed manga (considering the anime was kinda rushed and didn't explained properly according to Asou-sensei). And it's why Saiki being demi is perfectly fits with his behavior so well, from his avoidant tendencies, his tsundere trait passed from his grandpa, to being unreliable narrator.
What is demi? Demi is short for either demiromantic and demisexual to identifying person who attracted only after their deep emotional bonds created with someone. And in this case, Saiki is both.
A good example is through Saiki's relationship with Teruhashi. At first, he judged her based on her inner thought. Yet, he actually admitted her hard effort for maintaining perfect girl facade, and respected her. Not only that, his love language is act of service, that is showed from both manga and anime (giving a gift that is her favourite, bringing her to infirmary when she was fainted, ambushing Saiko's place with Kokominzu after he was threatened her from rejection, dragging three suitors away in her dating mixer, and so on).
Sure, Saiki can treat her platonically like the rest of his friends, but the story pushed he did have love feelings, but still in denial. And it's already clear 100% from the end, he admired Teruhashi's beauty before regained his powers back. I couldn't point it out when his feeling formed, but I can called it as slowburn, that apparently take a long time as his feeling built up through their friendship.
Now I know a lot of you guys probably said, "I-It's already clear Saiki didn't want to have relationship with other," "But I don't see Saiki EVER dating anybody", "Saiki didn't interested with romantic stuff!"
Let me show something shocking to you guys.
You see this? He using psychometry to find out the rest of the two alumnis love story when he was "trying" to finish the mystery novel. That's right, he acts like he doesn't enjoy romance stories but he invested in which is polar opposite from what he said.
In conclusion, a lot of interpretations claimed Saiki is aroace is unfortunately can be refuted. To be fair, it's alright and here is wrong to believe that headcanon, but it can reinforce the stereotype that could harmed Saiki as character himself and reduce him as "nonchalant pyschic who never cared about people around him", that is completely wrong about who he is.
If you guys have different opinions, let me know! ^_^