Herbert Melliphant is a pretty minor character, barely appearing in two chapters, but his actions set the novel's plot into motion. He really deserves more credit. And by credit, we mean blame for being a gutless weasel who ends up destroying the entire city.
We first see Melliphant in the first chapter when he teases Tom for being working class and having dead parents. Ah, nothing's funnier than orphans, right? He brings out the worst in Tom, prompting him into violence. As punishment, Tom gets sent into the Gut, the very same Gut where he meets Hester for the first time.
Much later, Melliphant is the guy with his "ear flattened itself against the wood of the door like a pale slug" (31.26). He overhears Katherine and Bevis's plan to blow up MEDUSA and runs and tells Magnus Crome, the man they're trying to thwart. Because Crome is made aware of the plan, he puts London on high alert, meaning Hester is captured alive, which leads Katherine to sacrifice her life for Hester's, which then causes MEDUSA to self-destruct, destroying all of London. Melliphant simply tipped the first domino in this intricate chain of events.
If Melliphant weren't such a worthless slime, we would have had a very short, very boring book to read, about some kid with a feather duster in a museum. Maybe we should thank Melliphant after all.