How do you think Kacchan will react/reacted when he sees/saw Deku’s body? I mean Deku probably has no muscle definition before inheriting All Might’s quirk.
I think, at first, Deku’s trying hard to avoid his body getting seen; that’s why he’s the last one to finish wearing his costume during their first battle training. I can also imagine him hiding away during P.E. when they’re changing in and out of their uniforms. Between those times and the Summer Camp arc, somewhere along the way Deku started feeling comfortable with his body.
The following images depict the first time he changed with the rest of the boys during P.E.
Warning: Lots of naked Deku, swearing Kacchan and awkwardness.
fadeslikewhispers 