For the first two half of Room, the only characters we meet are Jack, Ma, and Old Nick. Old Nick is the man who kidnapped Ma when she was nineteen and locked her in a shed in his backyard. He rapes her regularly. He brings her vitamins and food. When Ma's escape plan works, and Jack escapes, Old Nick gets caught and will probably spend at least twenty-five years in jail.
We never learn his real name. Jack calls him Old Nick because of something he saw on TV, event though "He doesn't look like the TV guy with a beard and horns and stuff" (1.105). That's right: Jack is pretty much calling this man the devil.
That assessment isn't far off. Whether he's the embodiment of pure evil or not, Old Nick clearly has control issues, and that's putting the problems of a man who would kidnap and lock up a woman for eight years very mildly. But he gets mad if he can't find what Ma asks for, and he manipulates her in any way he can, for example by getting Jack presents or by turning off the power in Room.
It's weird to realize that Jack is Old Nick's son, because Old Nick doesn't seem to care for him at all. Sure, he brings Jack clothes (that don't fit) and toys (okay, these actually work). He's really bad at taking medical care of Jack, but medical care is expensive, and this is a man who just lost his job. He's also a man who doesn't actually care about anyone other than himself, though, so let's be real: the only reason Jack is still alive is because Ma takes such good care of him.
So we don't sympathize with this guy at all. He's a violent kidnapper and a rapist. But we do have to wonder, just how did he get that way?