My favorite Haikyuu characters and what I think their biggest fear is…
Kenma- Ok so while Kenma doesn’t exactly have social anxiety, I personally feel he might have some sort of “social phobia” to some degree. He seems to always go out of his way to avoid having bad social interactions (so much so the guy literally dyed his hair for that reason alone) and he did mention that he cared very much what people thought of him, so him having social anxiety to some degree would be a possibility.
Oikawa- This dude has arachnophobia lmfao you can’t tell me different. Like he just seems the type of person to absolutely hate bugs in general. Poor Iwaizumi always ends up having to save him from the little spiders scuttling around the gym. Once Hanamaki tried playing a joke on him by throwing a plastic yet realistic looking spider at him from behind. This caused him to twist his ankle by accident from the way he fell down as he screamed trying to get the spider out of his hair. Poor guy lol.
Iwaizumi- Ok he definitely seems the type to not be afraid of many things but I think he hates needles. The poor guy gets so nervous whenever he has to go to the doctor because he always gets scared that he’ll have to get shots. But as Oikawa is the only person who knows this about Iwa-chan, he’s always being asked to go with Iwaizumi when he has an appointment to ease his anxiety (which ofc he’s more than willing to ❤️).
Atsumu: Snakes lmfao this guy hates them. I think it’s partly because he had bad experiences with them though. I imagine that once when he and Osamu were younger they went to the zoo and Atsumu accidentally stepped on a snake that got loose and ever since then he’s been scared and grossed out by snakes. Now when he goes to the zoo he avoids the snake section like the plague.
Osamu: I’ve thought about it and idk for sure why but I think he’d have a fear of heights, like he just absolutely hates it. He’ll never go to an area that’s higher up if you paid him. Once when they were 7 he and Atsumu went with their parents to a hike that had a glass bridge going over a large canyon that was over 40 feet tall, and Osamu absolutely refused to cross over it (he won’t admit it now but he might’ve cried also). Atsumu had to hold his hand and cross it with him (and he kept his eyes shut the whole time lmao). I also feel he has the bottem bunk bed just cause he doesn’t like the height of the top bunk bed lmao.
Kita- Personally I feel like Kita would also be afraid of heights. He avoids stuff like Ferris Wheels and ladders. His fear isn’t as big as Osamus but all the same he doesn’t like it.
Sakusa- Ok so we all know that he has an issue with germs but I feel like he’d also have claustrophobia. Like he can’t do elevators (not by himself anyway lol). I have a headcanon where Atsumu found this out about him because they got stuck in an elevator and Sakusa almost had a panic attack from the fear (Atsumu had never seen him this way since he’s usually cool, calm, and collected).
(I’ll probably add more but this is what I came up with so far lol)