Kathryn, this is a good question and it's helpful to remember that the primary function of a Nemesis is to provide opposition to the Protagonist. Thus, while White Mike doesn't officially appear until about 80 minutes into the movie, there is a combination of narrative elements which provide opposition to Jean and, thus, act as a kind of surrogate nemesis dynamic:

-- White Mike's minions: While not White Mike himself, there are thugs who show up from time to time, acting as a threat to Jean and her son.

-- Random characters: Such as the state trooper who interrogates Jean and Cal. In that scene, the trooper functions as a nemesis figure.

-- And as noted in my previous comments, the state that Jean exists in for pretty much the first half of the movie whereby she doesn't know what the hell is going on. That ongoing mystery tethered to the threat at that at *any moment*, she could end up dead at the hands of White Mike and his minions creates an opposition dynamice throughout.

I call this type of story structure whereby the real Nemesis doesn't officially intersect with the Protagonist until late in the story a Relay Nemesis Story. You know how in relay track races, the runners carry a baton and pass it off to the next runner? Think of the baton in this type of story scenario as the nemesis function. It gets 'passed' from one characte to the next, each creating opposition and, thereby, threat to the Protagonist... until it *finally* reaches the real Nemesis.

And specifically to your question: The fact that White Mike's minions appear in Act One, that is how White Mike (Nemesis) is introduced in the story.