That status was cemented on Tuesday when the former New York Police Department officer hand-delivered a letter from the President firing FBI director James Comey, entering the FBI headquarters with a manila folder and leaving without it.
Schiller, a White House official told CNN, went to the headquarters expressly to fire Comey, even though the FBI is currently in the middle of an investigation into Russia's meddling in the 2016 election and ties the hackers may have had to Trump associates.
Schiller is widely regarded inside and outside the White House as the "most underestimated person on Trump's team," as one former aide put it -- someone with constant access to Trump who has not only developed a deep level of trust with the President, but also has turned into a sort-of Trump translator.
During the campaign, "it mattered more to me what Keith Schiller thought of me than what the campaign manager thought," a former top campaign aide told CNN about Schiller. "Now I think it means more what Keith thinks of you than Reince (Trump's chief of staff) thinks of you."
Schiller joined the Trump Organization as a part-time bodyguard in 1999 after an assistant district attorney put in a good word for him with the real estate mogul. Formerly a NYPD detective who served in the Navy, Schiller quickly moved up the ladder with Trump, from temporary employee to bodyguard to Trump's head of security in 2005.
Friends from high school say Schiller never shied away from a fight during his younger years, a trait that has stayed with him. When protesters gathered around Trump tower during the campaign, Schiller ripped a sign out of one man's hands a punched him when he tried to grab it back.
Schiller, former aides and advisers said, understands the President better than most in the White House. During the 2016 campaign, aides would go to Schiller to understand a quirk or eccentricity about the billionaire-turned-politician.
"Any time I wanted to understand something, I would ask Keith," former Trump campaign manager Corey Lewandowski said. "I valued him for all the roles he played."
And he has played many roles for Trump: Press wrangler, body man, private muscle and sounding board. He is far more involved in day-to-day decision making than most heads of security. Journalists who interviewed Trump in 2016 have said Schiller is regularly the aide checking the quality of the shot to make sure his boss looks good.
Schiller, unlike Trump, is media-shy and has declined multiple interview attempts by CNN. When asked about his visit to the FBI headquarters on Tuesday, Schiller declined to comment.
Trump's management style has also benefited Schiller. The President regularly asks people close to him for their views. Schiller is often the person next to Trump, meaning he regularly opines on politics, policy and everything in between, one aide said.
But more importantly, Schiller enjoys something that not many of Trump's White House aides has: Trust.
This has been evident throughout Trump's few months in the White House. Schiller almost always travels with not only the President, but his top aides and children. When Jared Kushner, Trump's son-in-law and top policy aide, traveled to Iraq with Marine Gen. Joseph Dunford, the chairman of the US military's Joint Chiefs of Staff, Schiller was at his side for much of the trip.
Lieutenant David E. Chong, now the public safety commissioner for the City of White Plains, New York, who used to be Schiller's boss at NYPD, said he understands why Trump for puts so much faith in the former officer.
"I don't blame him," he said. "I would have kept Keith with me forever if I could have."
He added, "He always had my back. I was always very comfortable knowing he was around."