He is the son of Carozzo Ronah and the half-brother of Berah Ronah/Cecily Fairchild. Though he is not blood-related to Meitzer Ronah, he was adopted into the Ronah Family because of Carozzo's marriage to Meitzer's daughter. Within the Crossbone Vanguard, Dorel is a Mobile Suit squadron commander and the rival of Zabine Chareux, the commander of the Crossbone's elite Black Vanguard squadron. (villains.fandom.com)

U.C. 0123. Mobile Suit Gundam F91 is Gundam creator Yoshiyuki Tomino's attempt to launch a new Gundam saga, set thirty years after Char's Counterattack. 
The story of Gundam F91 revolves around teenage space colonist Seabook Arno, his friend Cecily Fairchild, and the efforts of the Crossbone Vanguard militia, led by Cecily's grandfather Meitzer Ronah, to establish an aristocracy known as "Cosmo Babylonia". 
In keeping with Gundam tradition, the civilian Seabook is forced by circumstance to pilot the F91 Gundam, coincidentally designed in part by his estranged mother, Dr. Monica Arno. 
(Source: Wikipedia)

Dorel Ronah