Izuku and Katsuki haven’t put a label over what they are, but the rest of the class has keyed in that there may be something going on there. Not dating, but definitely partners, in the sense that one follows wherever the other goes, not only in combat but in life. Close friends, but in a silent passion that isn’t present with Izuku and his other friends. Siblings, but the way they behave is little too affectionate to just be a kinship.
Izuku is fully aware of what he wants to be with Kacchan. Kacchan, however, does not seem to be getting the message, no matter how many times he invites him into his dorm room, or they spend solitary and tender moments together in the school garden. In fact, Kacchan, for whatever reason, is either ignoring it, ignorant, or leading Izuku on just for fun.
One moment, Kacchan will be sharing his bento with Izuku, squished underneath the shade of the giant tree in the courtyard, chuckling about stupid little things and nostalgia. He’ll look at Izuku with something like endearment, and a smile that isn’t going to bite his head off. He’ll shove little octopus sausages into his mouth and laugh when Izuku sputters and chokes on it. And in that moment, Izuku will feel like he’s on top of the world; Cherry blossoms littering the grass, the spring breeze running through his hair and dress shirt as he spends the next 40 minutes alone with his (second to his mother) favorite person in the whole world. Like nothing could top that, like this could go on forever and everything fades into the background; Jirou chasing Kaminari across the courtyard, the over looming threat of the Villain Vanguard, the heavy responsibility of becoming All Might’s successor.
But that illusion is immediately shattered when Kacchan forgoes a playdate together to go do.. Stupid, immature and probably against the rules things with his friends. Or when he brushes off all of Izuku’s attempts to grab his hand or work together. Or when he stares at Kirishima a little too long. Or when he laughs at Kirishima’s jokes a little too loudly, because there’s no way in hell that joke was actually that funny--And, okay, maybe it was a little funny but not enough to warrant the beautiful guffaw from Kacchan’s mouth..!
...Okay. So. Confession time; Midoriya Izuku sometimes got a little possessive and jealous over his not boyfriend.
But it’s understandable! When he’s got a catch nearly secured in his grubby little paws, he can’t help but want to snatch him up for himself. Away from Kirishima--(it’s really nothing personal.) But he can’t go about this.. Situation the way he wants to. He can’t just bag him up like a hostage. Their relationship, while leagues better than it was, say, three years ago, can still be a minefield. The hostile screaming matches have dulled down to just fuming in silence, but Kacchan can hold a grudge, and Izuku is prone to bouts of pettiness… (Not his best attribute.)
Kacchan isn’t socially aware of… Anything. It’s simultaneously the best trait, and the worst. The best, because when a cute and strong girl flirts with him, he doesn’t even acknowledge it--Not because he’s purposefully deflecting her but because he’s completely clueless. The worst because when Izuku pulls the moves on him, he doesn’t notice, or chalks it up as Izuku being a sappy nerd. (When Izuku bought him a limited edition All Might figurine that Kacchan had wanted ever since he was four, he was going for romantic nerd. )
This social unawareness can only mean a few things. Kacchan is not attracted to Kirishima, and he’s just good friends with him. The reason he pays rapt attention to Kirishima’s peculiar story about bathing in grape jelly is because he’s utterly stupefied at his friends antics.
OR--It means the exact opposite. Kacchan is paying rapt attention to Kirishima’s peculiar story about bathing in grape jelly because he also wants to slide around in that sticky tub with his good friend. Naked. Chest to chest. Bare.
Either way-- Izuku can’t make out whether straight up confessing to Kacchan would be a good idea. The possibility of getting his heartbroken is very high, whether it be because Kacchan just isn’t into him that way, or because he’s been fucking Kirishima behind all their backs.
He can practically imagine the disastrous confession, Kacchan responding, “ Stupid Deku! I’ve been fucking Kirishima all along, and he gives head better than you could ever! Go cry and jack off to those perv shots you take of me! ”
Fine! Izuku thinks, covertly glaring holes into the side of Kirishima’s head. Kacchan and Kirishima are having an animated discussion over some shounen manga, and the redhead looks close to just jumping out of his seat in excitement. Izuku narrows his eyes at the boy, but drinks in the twinkle in Kacchan’s eye as he talks on about his favorite character. His heart beats harshly and he turns his head away to keep it from just leaping out his chest and kissing that cute grin on his face.
If Kacchan wants to be gaga about Kirishima, that’s just fine. He’ll just have to make him jealous, give him a good wake up call and show him what he’s missing out on.
Thus, Midoriya Izuku sets in place his best mission yet--Mission: Make Kacchan Jealous/Mine.