A Look at Gen. James Mattis From a Close Friend

— -- When Gen. James Mattis retired from the Marine Corps in 2013 after a 41-year career, he planned a cross-country route back to his home state of Washington from his friend’s country cabin.

“The route he chose was dictated by the Gold Star Mothers he had to visit along the way,” says his longtime friend, retired Marine Maj. Gen. Michael Ennis, who first met Mattis in 1972, when they were both new officers. “He spent almost two weeks on that cross-country drive, zig-zagging to get to as many as he could to personally talk to them.”

“He personally wrote hand-written notes to every parent whose son or daughter was killed under his command,” said Ennis. “He cares deeply. He also works very hard to keep his Marines from being killed unnecessarily.”

Ennis described Mattis, who is a contender to be Donald Trump's pick for secretary of defense, as a "classic warrior" who "fights to win."

"He cares deeply about those who serve under him and the parents of those who die under him,” Ennis added.

Ennis lived with Mattis for six years during three separate assignments and shared with ABC News via email his personal insights into the storied general.

If Mattis is selected by Trump to become his nominee as Defense Secretary, it would require a waiver from Congress. By law, a civilian cannot be named to head the Defense Department unless there is a seven-year gap from the time they retired from active military duty. Mattis has only been retired for three years.

Ennis experienced first-hand how Mattis earned the nickname the “Warrior Monk."

“Jim read an astonishing number of books with the discipline of a monk,” says Ennis.

When they were both on recruiting duty in Milwaukee in the mid-1970’s, Ennis recalls his roommate staying home at night to read about World War II naval battles and decision-making.

“When I would come home, he would discuss critical decisions made by commanders,” Ennis recalls. “He would present a case and ask me to challenge it.” Mattis would then ask Ennis to support the opposing view. “It was a mental exercise for him; one that honed his ability to think critically,” said Ennis.

“He is an avid reader of all history, not just military,” Ennis says of his friend, who is believed to have a collection of 7,000 books.

During a visit to Mattis’ home at Camp Pendleton, when Mattis was in command of the 1st Marine Division, Ennis went to his friend’s kitchen looking for a box of cereal. “When I opened the kitchen cupboards, I was a bit taken aback when I saw that all his cupboards were filled with books,” Ennis remembers.

“I took a few out and opened them -- and saw notes he had scribbled in the margins and yellow highlights over interesting passages,” says Ennis.

He also says Mattis’ study of the warrior spirit goes beyond what he’s learned in books and personal experience.

According to Ennis, Mattis once visited Native American tribes during a vacation “talking story, and learning." “He is a warrior - one who wants to win; one who wants to protect; one who wants to preserve,” says Ennis.

But his friend also knew the importance of having fun. “I won't get into what we did, but just suffice it to say, he knew how to party and have a good time,” said Ennis. “He dated, he partied, he worked hard and he read a lot. He was well balanced.”

Mattis’ philosophy of working through assignments he was not keen about, particularly those in Washington, was "Bloom where you are planted.”

Ennis recalls his friend saying “that all he could do in an unpopular assignment was to do the best he could and clean up his portion of the world.”

And Ennis believes those Washington assignments and his stints at U.S. Central Command and U.S. Joint Forces Command make him qualified to be the next defense secretary.

Mattis has earned another nickname, “Mad Dog," for his tenacity on and off the battlefield, and that can include blunt comments he has made about the nature of war that have sometimes raised eyebrows.

Ennis says Mattis’ blunt talk shows “he is completely apolitical. He just tells it like it is.”

In February 2005, the Commandant of the Marine Corps “counseled” Mattis for public comments he had made about fighting the Taliban in Afghanistan. “You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil. You know, guys like that ain't got no manhood left anyway,” said Mattis.

"So it's a hell of a lot of fun to shoot them. Actually, it's a lot of fun to fight. You know, it's a hell of a hoot. It's fun to shoot some people. I'll be right up front with you. I like brawling."

Marine Commandant Gen. Mike Hagee said Mattis agreed that "he should have chosen his words more carefully." Hagee added in a statement, "While I understand that some people may take issue with the comments made by him, I also know he intended to reflect the unfortunate and harsh realities of war. Lt. Gen. Mattis often speaks with a great deal of candor.”

According to Ennis, Mattis can adapt to any audience whether it is when speaking to his troops or at a congressional hearing.

“It is like listening to two different people,” according to Ennis.” He adjusts his entire vocabulary and mannerisms to the audience he is addressing."

It's an approach Ennis believes embodies the line from Rudyard Kipling’s poem “If— ” -- “If you can talk with crowds and keep your virtue, Or walk with kings - nor lose the common touch.”