Akashi:
With his network and knowledge, Akashi would have already disposed of the love letter before you even have a chance to read it. If it does manage to slip past him, he'd simply ask you to hand over the love letter, and proceed to silently skim over the contents of the letter. He would then proceed to toss it into the nearest wastebasket, making sure everyone is watching as he does so. Or maybe, he would read the love letter out loud, adding in a few of his own snide comments to purposefully embarass and humiliate the sender.

"An elementary school student could compose a better love letter than this, don't worry _____, I shall take it upon myself to write you a love letter every day."

Aomine:
His eyes would narrow (even more so than usual) at the love letter in your hands, he wouldn't take action until he deems necessary. Of course, he would think that you wouldn't dare to think about any guy other than him. It is only when he sees your cheeks burn with delight and excitement that he takes action. Grabbing the love letter from your hand, he would throw away your love letter. Aomine would grab you roughly, pinning you suddenly against the lockers - a steamy makeout session is in his mind - to discourage any other guys bold enough to give you a love letter. He'd glare at some likely candidates and spend the rest of the day by your side.

"The next time I see a love letter in _____'s locker, I'm going to track the motherfucker who did it and beat the shit out of him."

Kise:
He'd respond quite positively, cheering and teasing you about the fact that you had fans too. He would be jealous but he wouldn't display it so easily, he would send menacing looks to the guy who sent you the love letter and be very touchy (even touchier than usual)- a subtle way of him marking his claim on you.

"I hope you know that I'll always be your number one fan, _____!"

Kuroko:
Being the observant little vanilla shake he was, Kuroko would see the person dropping off the love letter at your locker. With his skill in misdirection and sleight of hand, Kuroko would simply pluck the love letter out of your locker and dispose of it in the nearest trashcan. He'd spend the rest of the day with you, running little errands for you and taking you out on a random date. Even offering to buy you something from the bookstore you two would frequent.

"Let's keep this our little secret, okay Reader-san?"

Midorima:
Ah, ever the awkward carrot. Midorima would scrunch up his nose at the love letter in disdain. Worry nips at his heart as he thinks of a way to counteract this sudden move from the random rival. Midorima pretends to be composed but you can see jealousy leaking out of the tips of his emerald eyes. Coming off as a little brash and a little more forward today, you would pleasantly be surprised to see Midorima unafraid to get touchier with you. The next day, you would find a love letter from Midorima with a lot of crossed out words but filled to the brim with his sincerity and love. The next few weeks would be him, fastidiously showering you with gifts and trinkets that would make you grin like an idiot.

"Here, this rabbit plush is _____ lucky item for today."

Murasakibara:
The purple dinosaur (Barney?) would grab the love letter and keep it away from your reach childishly. Then, proceeding to rip it up and throw it away. Sending a death glare in the direction of the male students he would stick to you like glue the whole day and you'd have to keep on assuring him that you like him the best.

"There's no way, I'm gonna let anyone take _____chin away from me."