Gabriel's evil twin sure does wreak a lot of havoc on Gabriel's town throughout the novel. Yet, when we first meet the guy, he's just a snot nosed kid with a penchant for stealing—he hardly seems threatening or dangerous at all.
In fact, Gabriel recalls when he met Finnigan out by the fence:
I remember my first sight of him—the sound and scavenger look of him—surrounded by summer; I remember the stillness of the day and the density of the air. Neither of us was older than nine or ten. I was skimming a car along the garden fence when Finnigan crossed the brink of my vision. (3.4)
Notice how he describes Finnigan as a scavenger, as though the boy's searching for something. Perhaps Finnigan isn't roaming around aimlessly, as Gabriel imagines, but knowingly hunting for Gabriel. Finnigan is often described using animal imagery, especially that of predators (check out the "Symbols" for more on this), and here—when Finnigan seems harmless—the author is already giving us clues about his true, dark nature.
Pretty quickly, the boys form a pact. In it, they agree that in order for one (Finnigan) to be bad, the other (Gabriel) must be good. Sounds simple enough, right? Again, we get a hint as to what's really going down in the language. Check out the pact as they form it:
"Yeah, reflections! The same, but different. Like twins—like blood brothers! And when you need something bad done, like punishment or revenge, you'll just ask me, and I will do it." (5.52)
It's not just that Finnigan and Gabriel are buddies; it's that they are two sides of the same coin, positioning themselves as a sort of balancing act between good and evil. But, of course, they're actually even closer than twins: They are actually part of the same person, two sides of Anwell's psyche. Because of this, pact or not, Finnigan and Gabriel are bound forever.
We get another clue about their bound nature when Finnigan talks about the fires that he created all over town. Instead of merely referencing their pact, he tells Gabriel:
"You lit the fires as much as I did. You told me, go out and burn. Besides, we're reflections, blood brothers, remember? What I do, you do." The cold wind had brought water to my eyes; I smeared them dry on my sleeve. I could smell, on the breeze, the earthen scent of him. "You remember that, don't you, Anwell?" (9.89)
While it may seem at the time that this is about the blood oath they've made to each other, the truth is that Finnigan holds Gabriel responsible for the fires because he is literally as much to blame as Finnigan is, since they share a body. We see this later on as well, when the constable names Gabriel (or Anwell, as the constable knows him) as the arsonist instead of Finnigan. It's frustrating when we first read it, though it makes perfect sense once we understand that Finnigan only exists inside Gabriel's brain.
You might have noticed that Finnigan slowly but surely influences Gabriel from punk kid into cold-blooded murderer. At first, Gabriel is uncomfortable with all the wrong doing that Finnigan starts, but after a while, he comes to terms with it. And when he does, we see Gabriel evolve into a darker, more twisted version of himself with each additional rejection he experiences.
Eventually, Gabriel is willing to stand up to Finnigan. He tells his dark side:
"You always underestimated me. You thought you made me harmless when you gave angelhood to me. You forgot that some angels are warriors. Where there's warriors, there's war. I will fight to the death. It's my duty. I am not afraid." (21.13)
Eek, right? Gabriel defends himself against Finnigan after he realizes that he doesn't need some dark alter ego to do his killing for him—thankyouverymuch—he can do it himself. Ultimately, though, he wishes to go further and actually get rid of Finnigan. Gabriel claims this is because he doesn't want Finnigan to ruin anyone else's life, but is this really the case? It seems awfully convenient when the bones were just found. Perhaps what's actually happened is that Gabriel no longer feels split in two: Darkness runs the show now, so Finnigan is no longer needed.