Character Analysis
Isaac Who?
Isaac's a tough little nut to crack, especially since he spends most of the early parts of the book doing things like "standing in the water looking peaceful" (1.5.12), and saying "nothing as usual" (1.5.6). Daisy even acknowledges him as "the one who gets left out of most of the action due to hardly ever saying a word" (1.8.1). Though at first she "barely notice[s] him" (1.8.2), she eventually comes to realize that "it's the ones who aren't yakking all the time who sometimes turn out to be worth keeping an eye on" (1.8.1). Which is what we'll now do.
Silent Caretaker
In subtle ways, Isaac is the one who really takes care of the family, and "while Piper and Edmond were busy watching over [Daisy], Isaac was busy watching over them" (1.8.2). However, unlike self-important Osbert, "he didn't do it in an obvious way" (1.8.3). The kid "accepted the things people did, without comment or judgment and maybe without being terribly concerned. Even his family seemed to interest him in an abstract way, like lab specimens he'd come to feel responsibility and affection for" (1.8.5). So he's a kind but odd duck, this one.
Are We Human, Or Are We… Animal?
"More animal than human" (1.8.6), Isaac also has an uncanny ability to track others and predict their movements. Perhaps this also explains why "with nonhumans he was completely different […] every fiber of his being was totally engaged" (1.8.8). Isaac spends most of his time engaging with the animal kingdom rather than his human companions, and even the "expression of polite distance he always wore for humans [was] replaced by something concentrated and alive" (1.8.8). In other words, people just aren't this kid's bag.
More Than Meets The Eye
Lest you think that all Isaac's good for is talking to bunny rabbits, fear not. When it comes down to it, he gets done what needs to get done. During the war, he "knew that his first responsibility was to survive, and to make sure Edmond survived" (2.5.12), and when the twins are at odds for the first time, "Isaac proved the stronger of the two. He turned the full force of his will on Edmond. Bullied him. He did what was required to make sure they escaped alive" (2.5.12). Pretty impressive, right? Isaac's not just the quiet type—he's the strong, silent type.
In the end, playing his hand this way divides him and Edmond, permanently altering their relationship as Edmond witnesses the massacre and Isaac does not. That said, Isaac "suffered for Edmond as much as he could suffer for another human being" (2.4.37), which, if we had to guess, we'd hazard might be more than anyone realizes.