It is tempting at times to define Becky Sauerbrunn more by what she is not than what she is.

The 33-year-old Utah Royals centerback is famously not a shouter on the field or a rabble-rouser in the locker room. She spends her down time reading, not partying. She is not as prominent a pitchwoman as some of her colleagues, and she hasn’t always been outspoken about politics. She has played 158 times for the U.S. national team and never scored a goal.

Yet when Sauerbrunn does raise her voice, or endorse a sponsor, or join a cause — all of which have happened increasingly often lately — everyone immediately takes notice. For not only is the humble St. Louis native the Americans’ best defender, she is in many ways the team’s soul.

“She is a true leader, even though she doesn’t know it,” said midfielder Sam Mewis, Sauerbrunn’s roommate for the Americans’ recent World Cup send-off game. “Every time she speaks in front of a group, what she says is the most important thing that gets said, because you know she wouldn’t speak up unless it really, really mattered. She just knows what the team needs, and she will always speak up when she needs to, even if it makes her a little bit uncomfortable.”

Sauerbrunn probably does know it, even if she won’t admit it. The proof comes every time she steps on the field.

Her composure and positional instincts have made her a perfect foil for all three of her principal centerback partners in recent years. When it's been Julie Ertz, Sauerbrunn has stayed home and let Ertz play her all-action game. When it's been Abby Dahlkemper, whose passing is great but whose marking sometimes isn't, Sauerbrunn knows where to be. And when it's been newcomer Tierna Davidson, Sauerbrunn has taught the 20-year-old the ropes in real time.

Sauerbrunn’s role as the back line’s rock is especially important in the U.S. team’s high-octane playing style, when as many as seven players at once get forward. She’s the first to put out fires when teams counterattack — and as the world has gotten better at women’s soccer, that has happened increasingly often.

Sauerbrunn has also taken to the trail of activism blazed by her national team colleagues. Two years ago, she was one of the leaders of the players union’s collective bargaining effort, and lobbied the Oregon state legislature from her home in Portland during a debate over an equal pay bill.

“Becky is the moral compass, and the one who is always looking out for the collective,” said Becca Roux, executive director of the U.S. women’s players’ union. “She’s not the loudest, she doesn’t speak the most, she doesn’t want the media attention, but … when she speaks, you listen, because you know she is educated, she’s thoughtful, and she’s not going to say anything until she believes in it.”

This past March, when the U.S. team paid tribute to influential women by wearing those women’s names on their jerseys, Sauerbrunn sported “R.B.G.” for Supreme Court justice Ruth Bader Ginsburg. The national team sent Ginsburg a jersey, and she sent back a note of thanks with a promise that she’d wear the shirt while working out.

“I do think that playing on this team comes with a certain weight when it comes to being in the public eye,” Sauerbrunn said. “We’ve been very privileged on this team to have this platform to have a voice that gets heard, and a lot of these issues are near and dear to our heart."

Sauerbrunn is well aware of the risks in that, but she believes they’re worth taking.

“Yes, this country has its issues, but I also feel that this country has the ability to confront those issues and hopefully to work through those issues,” she said. “So when I represent this country, it’s knowing that it’s a great country, but it’s also a country that has a lot to work on. I’m willing to be a part of that [and] put the work in to make it as good as it can be.”