Rey Alfonzo was a Federation Starfleet officer assigned to Deep Space 9 in the late 2370s.
In June 2376, Lieutenant Alfonzo collected sensor signatures of Ambassador Seljin Gandres and Hiziki Gard when they were welcomed to Deep Space 9 by Colonel Kira Nerys. He then escorted Gard to the Trill delegation's quarters. (DS9 - Mission Gamma novel: Twilight)
In December, 2377, following the destruction of the Ascendant invasion fleet over Bajor, Alfonzo crewed the USS Volga along with Ensign José Chavez to intercept the lone surviving Ascendant Raiq. (DS9 novel: Ascendance)
In 2383, Alfonzo was killed in the destruction of Deep Space 9. (ST - The Fall novel: Revelation and Dust)