Maggie is pretty much a given in this category – we sympathize with her, the bulk of the novel tracks her personal story, and we have the greatest insight into her thoughts and feelings. She has protagonist – and a tragic one to boot – written all over her.
As with Maggie, we follow Tom’s story closely, and get a lot of sympathetic insight into Tom’s personality. The story may be mostly Maggie’s, but a large part of it also belongs to Tom. Tom is undeniably a punk. And he’s often downright antagonistic to Maggie. But, protagonists don’t have to be consistently likable. Crucially, the narrator often seems very sympathetic towards Tom and never really casts him as a villain, even though he and Maggie are often at cross-purposes. In a sense Tom is a sort of parallel protagonist to Maggie – the two often clash, but we often root for both of them. And their ultimate fates are highly entwined.