That Person is the eighth chapter of Hideyuki Furuhashi and Betten Court's Vigilante - My Hero Academia: Illegals manga.
Pop Step arrives at Kouichi's apartment while Kouichi himself is busy fixing his hoodie due to it being ripped. After grabbing a pudding, Pop Step wonders why Kouichi doesn't replace it with something new. Kouichi refuses to since it is official All Might merch. Kouichi tells Pop Step that he was planning eating the pudding, which makes Pop Step blush and causes her to blame for not labeling the pudding, although Kouichi doesn't mind. As Kouichi remarks on buying more pudding for her, Pop Step is annoyed that Kouichi spends more time on his lame All Might hoodie, causing Kouichi to snap at Pop Step, refusing to allow anyone to insult his All Might hoodie since it means a lot to him. Kouichi shows every limited edition All Might hoodie he has except for the silver age edition; the reason he is missing the silver age edition All Might hoodie is because he gave it so someone.
Kouichi explains that until middle school, he was serious about becoming a Hero and signed up for a Hero course at a high school in Tokyo. On the way to the entrance exam at the high school, as he is heading for the high school, Kouichi spots a child drowning and tries to ignore it, but his Hero instincts kick in and he jumps in to save the child. Kouichi saves the child and takes him out of the water. Seeing that the child is cold and wet, Kouichi gives the child his silver age edition All Might hoodie and rushes to the Hero course at the high school. However, Kouichi didn't make it in time and didn't take the examination. From time to time, Kouichi still thinks about a future where he became a Hero if the incident didn't occur. Kouichi hopes that one day, the child he saves grows up to be a Hero who was inspired by being saved by his heroics and hopes his ideals reach the boy through the limited edition silver age All Might hoodie he gave him. Unimpressed at Kouichi for messing up his Hero career so that he could rescue some nameless child, Pop Step goes home in a bad mood.
At home, Pop Step takes out the limited edition silver age All Might hoodie, revealing that Pop Step was the "boy" Kouichi saved on the day of his examination, causing her to blush. Pop Step thinks back; on that day, Pop Step was trying to use her Bound Quirk to jump across a river as a shortcut, but it had been raining the previous night, causing the ground to become wet and slippery and the river to overflow. These factors caused Pop Step to fall into the river and start drowning, but was saved when Kouichi came to her rescue. After that event, Pop Step couldn't get Kouichi out of her mind and has been looking for him ever since, and although she has finally met him, she doesn't know how to properly thank him. Pop Step becomes annoyed at Kouichi for thinking that she cannot apologize.
After getting pudding to eat and flying towards Kouichi's apartment, Pop Step is grateful for Kouichi for saving her and is regretful that she made him miss his exam. Pop Step knows better than anyone else, that Kouichi could have become a great Hero because Kouichi has always been a real Hero to her. Pop Step returns to Kouichi's apartment and wants to talk to Kouichi about something.
However, Knuckleduster's presence ruins her opportunity to tell Kouichi that the boy he saved was actually her. Knuckleduster eats the pudding while Kouichi asks Pop Step what she wanted to talk about. Annoyed at the inconvenience, Pop Step once again goes back home.
In her home, Pop Step puts on the limited edition silver age All Might hoodie, still thinking it is stupid that courage overflows from the hoodie according to Kouichi's belief. Although she thinks it is dumb, deep down Pop Step believes there to be some truth in Kouichi's belief.