Ironically I think approaching aro Saiki from the perspective of "this wasn't intended to be canon but let's seriously consider it despite that" is more conducive to actually exploring the concept in an interesting way than just going into the tag and angrily complaining about the fact that people ship him despite him being so "obviously aroace" and "basically the most aroace character ever" because he said that he didn't understand romance at a point in his arc when he was deeply dehumanizing himself and extremely pessimistic about his ability to have successful relationships with other people because of his nature and how he needs to keep it secret in order to live the normal life he so desires, even though the narrative goes on to push him in the vague direction of eventually entering a romantic relationship with Teruhashi Kokomi as the series goes on and he starts to open himself up to the possibility of friendship and acceptance and starts to consider himself neither above the need for human relationships nor inherently unlovable.
But I'm not actually all that interested in TeruSai despite it clearly being the mangaka's preferred pairing so let's circle back to the interesting discussions that are possible when we intentionally ignore this to consider the possibility of aromantic Kusuo. How it would make his avoidance of romantic situations not only the product of not wanting to enter a romantic relationship built on a lie but also because he doesn't feel like he'd be able to love them the way that they want him to (also possible with ace Saiki especially in the context of his relationship with Aiura). How he might consider his lack of romantic feelings another way that he's failed to be a proper human being and how he might feel relieved to learn that this isn't an experience that's exclusive to him as an esper and that there are other people out there who feel the same way who he can talk to and bond with over it. How even though he's probably glad to be able to avoid the hassles of romance he might still feel a bit bitter about missing out on romantic love which society (not to mention his own parents) hype up as the deepest, most profound and most valuable type of bond that two people can have with each other and how he might feel somewhat unmoored in adulthood once his friends start to settle down to start families and don't have as much time for him (though they really like him and he's good with children so I'm sure they'd let him be an honorary fun babysitter uncle). How it effects his relationship with gender when so much the concept is for the purpose of grouping together opposite gendered pairs for the production and rearing of offspring (though I suspect that his asexuality would have a greater effect for him since it seems like men are more expected to want sex than romance while the opposite is true for women). How he'd navigate his relationship with Teruhashi Kokomi who he deeply respects and enjoys the company of as a friend even if he doesn't return any romantic feelings she might have for him (don't hate on her even if you think he's aro. It's a perfectly reasonable assumption for her to think that he loves her when pretty much all other men including her own brother do and he's never told her that he doesn't because he doesn't want to hurt her feelings and is a little worried that the kokomins would have his guts for garters if they ever learned that he broke her heart).
