How?

Kotlers actions are mocking in that he consistently flirts with and seeks out the company of two women who are "off" limits. Gretel is too young for him..... but Kotler continues to flirt and show her special attention. It is safe to say that Kotler enjoys the attention of another man's wife and daughter, almost as if he simply flouting convention. We can infer that he has an affair with Bruno's mother by his actions during Father's absence. Bruno's observations easily lead us to understand that Kotler had the audacity to actually move in during this period, even if Bruno doesn't catch on completely. What Bruno does understand is that the man is around too often..... that Kotler "directs" his mother's decisions, and that he is far too familiar. We, as readers, are thus left to figure the rest of it out on our own.