The letter X. I know that only one of whatever the answer is, is in 24th place behind some other things so immediately I started thinking about days of the month, and the alphabet. Months did not make sense but I found that X is the 24th letter. Then to test the other parts, I added another X for 'a friend' and I already know that XX is 20 in Roman numerals. And when I add another x to make XXX, that's like an adult inappropriate movie rating, or 'unclean'. Did I get it right?