Without Gregor's income to support them, his family struggles to provide for themselves and must rent out rooms in the flat to increase their household income. The boarders are the three highly conventional men who rent out the rooms. Though the middle boarder appears to be their leader, the three men are ultimately nearly indistinguishable. They each have long beards, fixate on maintaining order in the flat, and are "painfully insistent on things being tidy." They essentially want everything to be organized to their exact satisfaction. Even when Gregor's mother kindly cooks dinner for them, they inspect the meat to make sure it is cooked to their liking before they are willing to eat it.
The boarders' presence in the flat furthers Gregor's isolation. Because they sometimes choose to dine in the living room, Gregor's door is often kept closed in the evening to avoid his being seen. Gregor’s family prioritizes the boarders' comforts, exhibiting "an exaggerated courtesy towards the three gentlemen" without daring to even "sit on their own chairs,” which highlights their desperation to keep the boarders happy as well as the extent of Gregor’s neglect. Illustrating this most effectively is the fact that the family begins using Gregor’s room as a storage closet once the boarders move in. They have, effectively, superseded Gregor’s role; where he once provided for the family, now it is the boarders who do so by paying rent.
The boarders do not hold the capacity for empathy. Once they learn of Gregor's condition, they are initially entertained, then grow angry with Gregor's family and assert that they plan to move out immediately without paying rent because of the “repugnant” conditions of the flat. The incident with the boarders is a major catalyst for Gregor’s family, Grete especially; she decides the insect can’t possibly be her brother anymore, indicating her compassion for him has faded entirely.