Crawlers who survived by taking a deal and now serve the dungeon as staff, like Milk.
When Milk mentions she was a Vesper before entering the dungeon, Prepotente seizes on it: "Wait. You're a former crawler? How interesting. This changes everything!" Milk's own record confirms it — Top Level 73, Dungeon Exit by taking a deal on the tenth floor, now indentured within the dungeon as guildmaster of the Calligraphy Guild and Cartography Guild.
The label lands on her as an accusation. She hears her own voice listing it out: you were a crawler, you fought, you were part of something once, and now all you do is sleep, you've missed entire seasons more times than you can count, you're a coward, your family would be ashamed. That self-indictment is what tips her into helping the pair anyway.
The class of dungeon staff Milk belongs to, and the identity Prepotente's remark forces her to confront.