It was clear as the sky is blue girlypop ٩(๑❛ᴗ❛๑)۶ First, "etchen" when he saw lulu in disguise as guild master ansur he immediately want her to lift her mask then when she did, etchen smiled and act like he knows her, and remember etchen/sid have the eyes that can see lies or smth like that i forgor, so no matter how lulu have skill to change her apparance (remember the time when lulu change her apparance to a grandma to nurse sid (and sid was weirded tf out why duke daughter doing that)) etchen/sid always know her disguise all along. Anw Later in the last panel (ch 94-95 where they met as guild master) where lulu gone, etchen/sid servant in the guild ask him why he agree to pay lotss of money js to see lulu face, etchen said its all worth it, & "no need to invistigate cz he alr knows her"— then the servant ask who lulu is and etchen said "she was my master" (〜 ̄△ ̄)〜 and b4 sid gone after being treated by lulu in dukedom, he said smth the line that lulu as his master (at ch 67 second to last panel)ヾ(❀╹◡╹)ノ~

Who is this etchen guy?
What chapter did he first appear? Coz to be honest, I am not the most consistent reader...