“Oh? Now this is a surprise. Are you sure you should have feelings for a thief, tantei? ~”
He’s teasing, but he really is happy to be getting the flowers from the teen. Well, aside from the lavender, but he isn’t surprised by it.
“Red can also mean passion, and I passionately don’t trust you.” This was not the truth, and his blush probably gave that away, but well, what could he do?

“Oh yes, I’m suuuure that’s what you meant, tantei.”

