Katsuki Bakugou loves izuku, more then anything in the whole world and he wants him to know. But its not like he can just march up to him and tell him how he feels, even after apologizing about all the shit hes done (If he ever finds the balls to do so) it would still be a pretty dick move to just come out of nowhere and go 

"Izuku. Ive always loved you, more then anything in the whole damn world!" 

In katsukis daydreams, izuku would react in two different ways:

"Kacchan. . . I.. love you too! I love you so so much!" 

Followed by tears and kisses. A happy ending. Or, 

"Kacchan. . . W-Why would you just say something like that? Is this another cruel joke? P-Please stop.. Im exhausted.. i cant take this anymore.. just leave me alone..." 

followed by him running away and Katsuki never seeing izuku alive again. If thats ever a possibility then katsuki cant risk it, not till he knows that option two is none existent. So hes come up with a different solution. Write izuku secret love letters until the day katsuki dies. So, thats what he'll do. Katsuki quickly goes to izukus shoe locker and slips the letter in and runs off. Izuku goes to change his shoes and grabs the letter, opening it. The outside is white with a red heart sticker keeping it closed, and the paper is fancy and of good quality. If katsuki was going to keep the sender of the letters unknown, its best if it seems nothing like him. The handwriting is in ink. Its font is neat and clean. Izuku reads it. 

Dear izuku, (Can i call you izuku?)

 Ive been seeing you a lot lately, i know that sounds creepy but who fucking cares. Do you care? Does that creep you out? Anyway, I guess you could say I've loved you for a while, more then a while actually, and i just wanted to say sorry. You dont have to know what for, at least not yet. I want to wait until im worthy of telling you all of this in person. This may sound random but fuck it i dont have anything else to say without getting too personal.  I really like when you wear those green hairpins to keep your bangs out of your face when your studying or at your desk. We arent really together a lot, at least in the way i want to be. God i hope you dont find that shit creepy either. Have a good day today. Keep this in your locker if you dont want me to write to you anymore if this was too much. Thanks for reading.

- Your secret admirer. 

Katsuki circles around and then walks up to izuku to see his face erupt in a wave of red blush. 

"What the hell are you doing damn nerd!? You look like a fucking shitty strawberry!" 

Izuku dazily shoves the letter into his bag quickly and mumbles an answer that katsuki doesnt really understand, then walks away. Katsuki watches izuku rush off and smirks to himself. Yeah. This works. This works perfectly. The next few hours are uneventful, katsuki doing his usual 'bully the quirkless kid who hes actually in love with' role and slips another note quickly into izukus locker and hides to see his reaction.