Rarely have the words "pretty" and "masculine" been used to describe the same person, but actor Song Joong-ki is an exception.

Song, who portrays the relentlessly romantic Yoo Si-jin in "Descendants of the Sun," has proven that pretty boys can be just as manly, if not more so than the Don Drapers typically cast in such roles.

In the KBS 2TV series, Yoo plays the Army captain who not-so-subtly admires Dr. Kang Mo-yeon (Song Hye-kyo). Even though his lines border on sexual harassment -- "Are you still sexy? In the operating room?" -- coming from the face of a model, they're perceived as irresistible.

In one scene, Yoo sneaks in a kiss with Kang before departing for Uruk, a fictional country where he is deployed for a peacekeeping mission.

"Should I apologize? Or should I confess my love?" he asks Kang later. To the confusion of many women, Kang demands that he apologizes, even though just like the rest of her gender, she secretly wishes the latter.

In some ways, Yoo actually mirrors a side of Song we've never seen, or never really bothered to. Ever since he debuted as No Tak, the sweet-looking guard of a Goryeo king in the movie "A Frozen Flower" in 2008, Song has been typecast in several eye-candy roles.

In the KBS drama "My Precious You," for instance, he was Jang Jin-ho, the sympathetic but not-so-bright younger brother of the female protagonist, In-ho. In the movie "Hearty Paws 2," Song's character, the soft-hearted Dong-wook, becomes attached to a white Labrador retriever and looks after her three puppies. Song's other support roles in the TV series "Triple" and "OB-GYN" have also capitalized on his effeminate features.

But none of these have done justice to the reality of Song's sporty and sharp persona.

A short track speed skater in his youth, Song represented his hometown Daejeon and competed in three national competitions between first grade and high school.

After being forced to abandon that career due to an injury, Song went on to prosper academically too, earning all A's in his high school senior year and entering the prestigious Sungkyunkwan University in Seoul as a business student.

"Descendants of the Sun," in that sense, simply provided an outlet for Song to demonstrate his inner masculinity. Feeding him with cheesy, cringe-inducing lines concocted by pop writer Kim Eun-sook, we get Capt. Yoo.

Another factor that has contributed to Song's marvelous transformation into an alpha male is perhaps the fact that he recently served in the military.

South Korea requires all able-bodied men to serve about two years in the military, a legacy of the 1950-53 Korean War, and "Descendants of the Sun" is Song's first TV work since completing his military duty in 2015.

Song has noted that he probably benefited from that timing. Asked whether he found acting easier in "Descendants of the Sun," he said, "that's what I found to be the case too, since I was already accustomed to the military manner of speech and had a military cut."

But he stresses there's more to Yoo than simply being a man in uniform.

"Because my character is so serious and has great leadership, it's becoming harder to portray those aspects of Yoo," he told reporters at a press conference ahead of the season premiere.

The self-aware actor admits he's "not the type of actor who is known for his body." But thankfully, he had had two months to get in shape before the filming started.

Meanwhile, leading woman Song Hye-kyo has confirmed what we have already suspected.

"I've received so much help from Song since he's a gentleman around his acting partners," she said at the press conference. "On long days, he would cheer me up. He's been such a reliable younger brother."

"What's more, there were many times when his acting inspired mine," she adds.

New episodes of "Descendants of the Sun" air every Wednesday and Thursday at 10 p.m. (Yonhap)