If Fisher, the head guard at the prison, were alive today, there's a good chance he'd be a pro wrestler. When Isabel first meets him, she describes him as "a mountain clothed in a lobsterback uniform" with "iron mallets" for hands (35.17). Yikes. While he might not sound like someone you'd want to run into on an illegal visit to a prison, Isabel figures out pretty quickly that she can get pretty much anything she wants by giving him food. He's pretty eager to "inspect" (35.34) her donations, and by "inspect," we mean eat half of it.
Fisher's largely a comic relief character. Seriously—what's not funny about a big, hulking dude who eats a ton and says things like "Wot you bring him today?" (40.11). Still, his selfishness lends extra meaning to the way the soldiers in prison share the food Isabel brings. "We fought together," Curzon tells Isabel. "We'll eat together" (35.82). It's an attitude Fisher could afford to develop.