Some say that the pitcher’s mound can be the loneliest place on earth, with so much resting on the pitcher’s rhythm, arm and accuracy. For Shelby Miller, a 23-year-old player on the roster of the St. Louis Cardinals, not being selected to pitch for his team in this year’s World Series put him in an even lonelier place.

And when he needed to vent about being left in the bullpen, Amy Peters was the one person he could express his frustration to. “She would remind me that I was good and that there was some reason unknown to us why I wasn’t pitching,” Mr. Miller said.

His postseason absence is not an indictment of his career. He placed third in the 2013 National League rookie-of-the-year voting. He is “definitely a young pitcher to watch,” said Kary Booher, sports editor for The Springfield News-Leader, in Missouri.

For the last two and a half years, Ms. Peters, 22, has been Mr. Miller’s most loyal cheerleader. They met in June 2011, when he began playing for the Springfield Cardinals, a minor league team; Ms. Peters was a member of its cheering squad. “We had to sign a piece of paper telling us we could not socialize with the players,” she said. “I’ve always been really bad at following rules.”