A reptilian super-being, possessing a hard exterior body structure and tremendous strength, Rex is the son of Madame Majestrix, one of the greatest enemies of the First Family and the Queen of Monstro City. Rex is a prince of Monstro City.
Rex is a second generation member of the First Family, sharing adventures with the group as early as 1973. He developed a strong attraction to one of the members, Natalie First, this in turn motivated him to spend a great deal of time with them before officially joining the team later on. The First Family had strong affections for Rex as well. Disgusted with how the government wanted to imprison (and probably intrusively study) Rex, the First Family self-exiled themselves to their orbiting space station, Outpost F.
His marriage to Natalie occurred sometime in 1979, the exact date not disclosed. The location of the marriage is not disclosed either, but the one panel showing the wedding ceremony suggests that it is probably the First Family's Headquarters, on Mount Kirby. His in-laws, Julius Furst and Augustus Furst are easily recognized in the background, along with a couple of shrouded figures who are obviously of the same species as Rex. There are other colorful characters, but the shadows keep their identities a pleasant, but frustrating mystery.
Not much is known about his mother or their emotional relationship with each other. In 1972, he led an army of Madame Majestrix's Monster Troops in an attack that was rebuffed by the First Family. A few years later, in Kurt Busiek's Astro City Vol.2 issue number two, we witness a battle that Rex fought against her and alongside the First Family for possession of the Gemension Jewel. Shortly after that, a species called the Trenchers invaded Rex's underwater homelands, forcing the citizens to flee further into the oceans depths leaving Monstro City abandoned and in ruins. Circa the later half of 1996, Monstro City remained in a state of ruins and abandonment. Its current condition is unknown.
Physical traits suggests a city-smart, rough New York Bronx bawdiness. In fact, a certain coarse, direct approach to people and situations has been demonstrated. His approach in battle is fists and muscle. He is a quick-thinker, never hesitating at taking charge and directing the scope of a battle in complex scenarios.
In non-battle situations, other traits can be readily observed. As mentioned earlier, he is a royal family member, and heir-apparent to the throne of Monstro City. So a certain sophistication and upper-class mannerisms should be assumed. On the other hand, his culture has traits of a warrior-based social hierarchy.
His preferred drinking establishment is Bruiser's, and his uncle-in-law, Julius Furst, usually accompanies him there. Given his noble heritage, it is assumed he can dress the part for Butlers as well, but since he can't conceal his identify he rarely if ever attends there.
The naming convention of Western civilization differs from that of Monstro City, and adapting Rex's name to fit it is no precise science. He has alternately been called Rex Zorus, Tyranos Rex, and Rex Majestros, and all three are valid titles.