In the LN (volume 9), he doesn't tell her because if he had revealed he was the guy that saved her, she might remember him as the fatass loser he was at the time, and his appearance back then might prejudice her against him now. "Appearances do matter, you know?"Like why even leave that up in the air? Characters leaving potentially consequential trivia about connections to other characters unnecessarily up in the air is my biggest pet peeve.