Fermín signed with the Kansas City Royals organization as an international free agent on July 25, 2015.[1] He made his professional debut with the Dominican Summer League Royals the following year, hitting .273 across 52 contests. In 2017, Fermín played in 47 games for the rookie-level Idaho Falls Chukars, slashing .282/.403/.356 with one home run and 39 RBI. He split the 2018 season between Idaho Falls and the rookie-level Burlington Royals, posting a cumulative .249/.346/.350 slash line with 2 home runs and 19 RBI.
In 2019, Fermín split the season between the Single-A Lexington Legends and the Double-A Northwest Arkansas Naturals, hitting .260/.298/.418 with career-highs in home runs (12) and RBI (41) in 86 games between the two affiliates. Fermín did not play in a game in 2020 due to the cancellation of the minor league season because of the COVID-19 pandemic.[2] In 2021 he spent time with Northwest Arkansas and the Triple-A Omaha Storm Chasers, batting .277/.355/.440 with 10 home runs and 47 RBI in 77 contests. He was assigned to Triple-A Omaha to begin the 2022 season.
On July 14, 2022, Fermín was selected to the 40-man roster and called up to the majors for the first time after multiple players were placed on the restricted list prior to a series in Canada against the Toronto Blue Jays.[3] He made his MLB debut on July 15, ultimately appearing in 3 games but going hitless in 7 at-bats. He was removed from the 40-man roster and returned to Omaha on July 17.[4]
On November 15, 2022, the Royals selected Fermín’s contract back to the 40-man roster to protect him from the Rule 5 draft.[5] Fermín was optioned to Triple-A Omaha to begin the 2023 season.[6] He began the 2024 season as the Royals' regular backup catcher.[7]