The worst of the worst rising to the top: Mr. Miller was influential in Mr. Trump’s first term but stands to be exponentially more so this time. He holds the positions of deputy chief of staff, with oversight of domestic policy, and homeland security adviser, which gives him range to coordinate among cabinet agencies. He will be a key legislative strategist and is expected to play an important role in crafting Mr. Trump’s speeches, as he has done since he joined the first Trump campaign in 2016. Most significantly, Mr. Miller will be in charge of Mr. Trump’s signature issue and the one that Mr. Miller has been fixated on since childhood: immigration. And he has been working, in secrecy, to oversee the team drafting the dozens of executive orders that Mr. Trump will sign after he takes office on Jan. 20. “I call Stephen ‘Trump’s brain,’” said Kevin McCarthy, the former House speaker who credited Mr. Miller — a private citizen at the time — with helping to rally Republican lawmakers to insert a sweeping border crackdown into a spending bill in 2023. In the four years since Mr. Trump has been out of office, Mr. Miller has spent more time than any close Trump adviser mapping out a second-term playbook. He expanded on the hard-line first-term immigration policies; he deepened his relationships with House members, senators and influential right-wing media figures; he built a nationwide donor network to fund a nonprofit that he used as an additional tool…