Yeah, we know he's not a villain, and we can't even say he's unlikable, because he loves Opal and tries to be a good person. But from Opal's perspective, "it's hard for me to think about him as my daddy, because he spends so much time preaching or thinking about preaching or getting ready to preach" (2.1). In fact, Opal scrambles to have a conversation with him before he retreats into his turtle shell.
Sure, the preacher is only keeping Opal from himself. And her mother. But here's the thing:those are the two things she wants most. Luckily, he doesn't stay the bad guy for very long.
No, they are not especially developed characters. No, they're not pure evil. Yes, they all end up friends. But these two pests shove a thorn in Opal's side (figuratively) throughout most of the book. They tease her as she pedals past their house, claim Gloria is a witch, and follow her on bikes, talking so low Opal can't hear them.