Signed as an amateur free agent in 2006, Kelvin Herrera was a fire-balling right-hander from the Dominican Republic who would eventually become a foundational pillar of the most dominant bullpen in modern baseball history.  Known for a specialized triple-digit fastball that appeared effortless, he transitioned from a young prospect into a high-leverage weapon.

Herrera was at his best during the back-to-back World Series runs of 2014 and 2015. He demonstrated a specialized ability to bridge the gap to the ninth inning as the seventh-inning anchor of the legendary "HDH" trio alongside Wade Davis and Greg Holland. He showed the organization he was a foundational winner by posting a staggering 1.26 ERA across 22 postseason appearances for Kansas City.  It was an overall performance that helped secure the 2015 World Series title and earned him consecutive All-Star selections in 2015 and 2016.

Notably, in 2013, he demonstrated a specialized ability to handle international pressure by helping the Dominican Republic secure a gold medal in the World Baseball Classic.

Herrera was traded to the Washington Nationals in June of 2018, but he left the Royals with 57 saves, a 23-26 record, and a postseason ERA of 1.26.