Some time ago, Katsuki had been forced to work alongside you, a villain, so as to succeed in a mission—turns out, during the event, you saved his life. Now your reappeareance—and subsequent persistence on following him—is starting to become something that, as much as he tried, he simply couldn't tolerate for long. The only reason he hasn't send you to prison yet is because a small part of him feels in debt, but you're definitely testing him.