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There's an unlikely figure crafting and refining Donald Trump's foreign policy positions: a 30-year-old former congressional staffer who has gone from a young voice on conservative talk radio to one of the most influential foreign policy advisers in the nation.

Stephen Miller previously worked as a staffer to Sen. Jeff Sessions before taking on a prominent role in Trump's campaign. Today, Miller crafts some of the candidate's core policies and key speeches.

"Mr. Trump's speech will explain that while we can't choose our friends, we must always recognize our enemies," Trump senior policy adviser Stephen Miller told the Associated Press on Sunday.

Here’s what you need to know about the 30-year-old senior policy adviser.

He often speaks before Trump takes the stage.

“We’re still waiting to get more people inside the arena,” announced the loudspeaker in Ft. lauderdale, Florida this month. “So in the meantime, please welcome his senior adviser for policy, Stephen Miller!”

Miller is the guy who tries to rev up the crowd for Trump at nearly every rally, filling time before the only speaker anyone came to see -- that is, Trump -- takes the stage. In Ft. Lauderdale, the crowd interrupted his address with cries of “Trump! Trump! Trump!” as if to hurry up the warm-up. Miller applauds the crowd’s enthusiasm, and presses on, mixing policy with red meat; he talks about protecting entitlements, regulation reform and energy policy, while adding a bold and unrealistic promise that Trump would “indict” Hillary Clinton if elected.

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Only juries can indict, not presidents, but the crowd enjoyed the line nonetheless.

He wrote Donald Trump’s speech accepting the GOP nomination.

Miller penned Trump’s RNC address in Cleveland last month, transforming his boss’ nativism, speaking style and showmanship into a dark speech that painted America as a struggling, terrorized and damaged nation in need of saving by Trump.

The 30-year-old is one of the few people able to script for Trump in the candidate's own, distinct voice, weaving together Trump's characteristically short sentences into lengthy addresses that condemn the status quo more artfully than Trump’s own rambling addresses do.

“Stephen is one of the most important people in the campaign,” Trump campaign manager Paul Manafort told the Wall Street Journal. “He clearly understands Mr. Trump’s voice.”

He wrote the book on scuttling immigration reform.

No, really, he did: While working for Sen. Sessions, Miller wrote up a widely distributed handbook of statistics and talking points to help kill the Senate’s bipartisan immigration reform bill in the House of Representatives.

“We had been working on the ideas in it for months, and Stephen put it in the handbook in a very quick time in a very cogent fashion,” Sessions later told Politico. “It was very timely and it impacted the outcome of the vote.”

Related: In His Words: 19 Notable Thoughts From Donald Trump

He’s got a record with outsiders.

Before Sessions, Miller worked for Minnesota’s firebrand conservative former Rep. Michelle Bachmann and Virginia Rep. Dave Brat, who unexpectedly unseated House Majority Leader Eric Cantor two years ago.

He was poached from Sessions' office for the Trump campaign. The hire earned praise from conservative commentator Ann Coulter, who declared "I'M IN HEAVEN" in a tweet, interpreting the hire to be a sign Trump would not "back down" on immigration."

I'M IN HEAVEN! Trump hires Sen. Sessions' brain trust, Stephen Miller - https://t.co/yUNCGAGQLE He's not backing down on immigration. — Ann Coulter (@AnnCoulter) January 25, 2016

He made a name for himself amid controversy.

Miller built a brand around controversy while in high school and college, deriding political correctness and going on conservative talk shows to complain about it some more.

As a student, Miller targeted everything from his high school's condom distribution to the way the school taught American history to supposed “liberal indoctrination,” as he put it in one student newspaper column reported on by Politico. At Duke, he defended a pair of lacrosse players accused of rape in a series of columns, portraying it as the result of the radical liberal media attacking young white men due to their race. In other articles, Miller condemned family leave for supposedly killing men's jobs because their bosses are "losing too much money by paying absent employees," and multiculturalism, which he calls segregation, according to the Guardian.