(Since I don't really ship minors and adults, because it's too freaking WEIRD, reader is going to be an adult in this. That is all)
You might either be the older sister/brother or heck, even a single parent or something to one of the Karasuno team members. or maybe even a teacher at the school for that matter. He'll meet you for the very first time at a volleyball match between Karasuno High and Aoba Johsai where he sees you cheering for your sibling/kid telling them to destroy those little punks. He's a bit surprised by your fiery nature but he's still a bit amused by it
He'll strike up a conversation with you later and on the inside, he's really glad and happy that his team has such an excited supporter. And bonus if you used to play volleyball when you were a kid, he'll offer you to play with him if you wanted to
And maybe after that.... he'll take you out for some coffee that day and the next day and day after that and so on. It's his way of trying to befriend you before trying anything with you. Takeda notices he gets a bit red and starts blushing a bit whenever someone mentions your name. And if you're a teacher in the same school, good thing, he'll come by your class randomly sometimes trying to pretend he has something important to do like handing our papers and stuff, but who is his man fooling? He's a complete simp for your voice and he has to see you again, else he'll not get a good night's rest
He gets visibly angry and pissed if someone talks nonsense about you and let me tell you, this man can punch like no one's business. In fact, he won't hesitate or think twice of slapping the living daylights out of someone for daring to utter nonsensical garbage about you. You should be thankful and glad he didn't beat them to a bloody pulp and land them in a coma or something. A broken tooth or a few broken bones is nothing too drastic compared to what he's planned to do with rivals. And speaking of rivals, he'll threaten them to stay away from you at first, he doesn't actually go on a full murder spree or something but if they're hard headed idiots and don't really care for his words, they'll be kissing the world goodbye in literally less than 5 seconds. How? He'll snap and break their neck like a twig. Have you SEEN the size of his arms?
As a yandere he is obsessive and highly possessive of you. He might also be a bit manipulative depending on the situation and he doesn't really show that much of remorse if at all someone lands in the hospital because of his rage and doings
Students who have a crush on you will be expelled and he'll make sure their future gets ruined and fudged. As for you being a single parent, he'll try winning your kid over first and then he'll start making his moves on you. Oh, he's quite the crafty one. And if you're the older sibling or someone, he'll find a way to make sure you stay with him at all times, either as another coach along with him and Takeda or maybe a manager or whatever. Point is, he will not let you out of your sight
Is he a stalker? You can bet your money he is. He'll follow you home to see if you reach home safely or not, only then will his heart and mind be at peace. The only time he'll kidnap you is if you're moving away from him or if you start going out with someone other than him (Not Takeda though, he's his wingman) he'll snap and do something completely abnormal and irrational like kidnapping you and having you for himself. He doesn't really have that much of patience if you struggle and resist his love so... try embracing the situation at hand with ease dear? Else he'll have to show you how much of a monster he can be when he's angry. And trust me, no one wants to see him angry, dude's a complete menace when he is
He likes physical affection, like A LOT. He loves having you on his lap, whispering sweet and no no things sometimes in your ears that will undoubtedly make you beet red with embarrassment but since he'll a cocky lil' shit, he just grins to his heart content lol
He promises to treat you well and give you all the love and affection you deserve, just please love him back. Things won't end up well for you if you don't.....