In episode 39, so spoiler warning, in the part where both of them are staring up at the moon, Tou Tou references a poem and talks about a line Tian Zhen spoke to her about by a Japanese writer; then she says, "The moonlight tonight is truly beautiful." According to this piece of writing, the writer says, "The moonlight tonight is truly beautiful" to say "I love you" to his loved one. So she's literally saying I love you, TianZhen at 13:15. In case anyone was wondering. It wasn't explicitly laid out by the show (or at least not by the YouTube subtitles, which had their fair share of mistakes).