@Jophvalles Yep! That is exactly what I was going to say that 반만 means informal speech. The opposite of 반말 is 높임말, which is formal speech.
Example of 반말 in English: Hi! What's up!
Example of 높임말 in English: Hello. How was your day?
P.S. We are refering to the guy who said, "...Huh?" in this chapter. In the raw, he says, "....반말?"
By the way, thank you for the translation and your handwork!