Kristina's stepfather may be more reliable and generally a better parent than her biological father, but he has plenty of flaws of his own. Kristina first describes him as "stern and heavy-handed, with unattainable expectations" (Alone.2). Ouch. Sounds like this guy not only has super high standards, he is quick to punish his kids if they don't live up to them. And in a story about Kristina basically rejecting these standards for a lifestyle of drugs, this especially can't be a good thing.
Obviously, Scott's not thrilled with the changes that begin unfolding in his stepdaughter's life. When Kristina ends up in juvenile hall, Scott "sulked like a pissed puppy" on the way home after picking her up. "He refused to say one word," Kristina says, "and his silence told me all I needed to know (Cause and Effect.3). To be fair, Kristina doesn't exactly deserve a round of high-fives at this moment.
When Scott begins building a solid case that Kristina's using drugs, he tells her mom that he wants to restrict all access to her friends and after-school activities and monitor her report cards. Basically, he's the kind of parent you don't want to mess with, or at least hope you don't get caught if you do. This dude means business—all the time.
Hang on a second, though. While Scott's anger at Kristina is totally justified (after all, what parent would be thrilled about their daughter being on drugs?), we have to wonder if there isn't more to his marriage than meets the eye. When she sees him at the airport, Kristina notices signs of stress in his face and wonders if they've been caused by a "hard day or another argument" (I Saw Them.4). Furthermore, at the air races, Marie shamelessly flirts with another guy while "Scott pretended not to notice" (We Found Our Box.2). Bummer, right?
In the end, it could be that Scott's high standards and expectations are an effort to defend his household and life against Marie's selfish streak. It's tough to say for sure, but we kind of have to feel bad for the guy.
Plus, though he keeps it fairly well hidden, Scott does have a soft side. While Kristina's pregnancy initially turns him cold toward her, she eventually finds him "losing his anger long enough to teach me to drive" (Highs.6). At least he recognizes that while she's about to be a mom, Kristina's still a teenager, and teenagers need driver's licenses. Or maybe it's that while she's still a teen, she's about to be a mom, and moms need driver's licenses. Either way, dude relents.