How did he know so much about you? You always wondered every time you saw something you liked in your U.A. bank with a letter next to it. Many had told you that it was always Katsuki who let you the letter's. At the end of the day you wanted to confront him if it was true and when you did he responded in a bitter tone.

"Why do you ask stupid questions? It is not I who leaves you such letters and gift's."

He was clearly lying, but Katsuki was a very gossipy person. he will have heard you, asked someone, searched or something else about you to give you things. Could it be that he is in love with you and wants to get your attention?