When monsters show up in literature, bad things usually happen. Think about the chain of events unleashed by Frankenstein's Creature, Beowulf's smash-up with Grendel, or Smaug's mass chaos in The Hobbit. Crank, too, features a monster of its own, except it's not a greedy dragon or a giant with mommy issues—this monster is meth addiction personified.
When you think about it, the monster is an apt name for the influence crank wields over its users. It's powerful, out of control, and terrifying, with the potential to transform people into something deformed and dysfunctional. For instance, when Adam describes his confrontation with Lince to Kristina, he tells her, "The monster rose up hard then, hard in her eyes. She looked like an animal, crazy mad, diseased" (He Told Me Why Anyway.4). While Lince has previously been described as stealthy and catlike, meth has turned her into something out of control, rabid, and terrifying.
Later in the book, Kristina describes the monster as a different kind of creature—a strangling, life-sucking octopus. According to her, the monster's capable of "weaving its tentacles not just around you, but through you, not hard enough to kill you, but enough to keep you from reeling" (Crank, You See.2-3), until you're actually addicted to its grasp. Like a giant squid, you recognize that meth is holding you captive, but are unable—and even unwilling—to get free. It's that powerful.