After Izuku moves into the dorms, All Might makes a point of coming to visit Inko with updates on her son. He knows how worried she is and how lonely she must be without Izuku so he tries his best to be good company and put her mind at ease.
For the most part this happens while Izuku is physically at UA, so he witnesses none of it. One weekend when Izuku goes home he invites Ochako over for dinner. Meanwhile Inko has done the same with All Might, who’s now entirely comfortable and casual in the Midoriya household, helping cook and chatting with Inko. Ochako watches this unfold like, “Uh Deku…Is All Might like…your dad now?” And Izuku is completely shocked like “Haha what no?? He’s just my mentor.”
Meanwhole All Might an Inko are clearly flirting.
Then Ochako leaves for the night and Izuku lapses into existential crisis mode because “…..shit……she totally has a point….he’s so casual here what is he now?” And Izuku picks up on all these small changes in the house just “Why is there an extra futon here? Whose toothbrush is this? Where did all these large shirts come from? There’s so much All Might merch around and it’s not my All Might merch who bought these?” And All Might pops his head in like, “Oh yeah, I get my own All Might merch for free.” And Izuku’s just….screaming inside.
Break rolls around and Izuku packs a suitcase to head home for two weeks. Walks in the front door and All Might is standing in the kitchen with a chef’s hat on baking. Izuku wanders in in a complete daze, All Might acting like this is totally normal as he pulls out bowls and spatulas while Izuku starts muttering, “Why are you…. here all the time…??? how do you remember where all the kitchen tools are when I don’t…???” and he glances at the partially-frosted cake on the counter, “Why are you making a cake that says ‘Welcome home son’????” And All Might starts blushing furiously like, “Well I was finishing this for your MOTHER.”
School resumes and all the Class 1-A kids are walking to class. They pass by the school grounds entrance and suddenly Inko is there running toward the main building. Bakugou notices first like, “Hey Deku, isn’t that your fucking mom over there?” Izuku turns, startled, and intercepts her thinking it’s got to have something to do with him. She just kinda blinks at him when he asks what’s wrong and says, “Uh actually… Toshinori forgot his lesson plans at home… Could you give this to him?“ Ochako, Iida, and Bakugou who are all listening internally go “Who tf is Toshinori…?” Until they all walk into class and Izuku sheepishly hands the papers off to All Might and the rest of the kids just “!!!!!!!!!!!!!!!!!!”
During class break Iida and Ochako look at each other like "ALL MIGHT WAS IZUKU’S DAD” They turn to izuku with gaping mouths like ‘ALL ALONG???’
Izuku flails a bit. 'I DIDNT - HES NOT - THIS!…”
“HOW LONG HAVE THEY BEEN DATING”
Izuku, still sputtering, “THEY AREN’T??? DATING????”
Iida nods like he understands and says “Well of course not Izuku. Why would they go on dates when they’re already married?’
Ochako answers “Not true my parents still go out sometimes.”
Izuku “WHY AR E WE DEBATING THIS”
They just keep having a conversation without Izuku. Izuku instead passes out against the wall muttering. Iida and Ochako finally notice. “Should we call his parent?” “Which one?”
They’ve got a class trip coming up and after lunch Aizawa hands out permission slips. Izuku’s is already signed with an enormous ALL MIGHT signature.
Since everyone seems to know now, it’s go big or go home.