Native open monosyllabic nouns in Russian are indeed scarce, but a few do exist. These include зло "evil," тля "aphid," дно "floor, bottom of surface" and сто "hundred." In the case of зло, simply an -й is added to the stem in order to derive the adjective злой, and in the case of тля, the я transforms to е and the suffix -нный is added, тленный. Both дно and сто make use of metathesis + the suffix -(н)ый: сотый and донный, respectively. All of these words were originally disyllabic: zъlъ, tълja(?), dъno, sъto; and some might argue that they are still pronounced disyllabically today, i.e. тля /t.lʲæ/ and дно /d.no/, although they are supposed to be pronounced monosyllabically /tlʲæ/ and /dno/.

The loanword чмо "schmuck" (from Yiddish) derives its adjective by the suffix -шный: чмошный. However, the names of the letters of the Russian alphabet are indeclinable, and so are most open monosyllabic loanwords, for instance па "pas, step" (from French), го "Japanese boardgame" (from Japanese) and пи "pi" (from Greek). As far as I am aware these do not have any derived adjectives, and instead other constructions are used (for instance a genitive construction). The same is most likely true for the name you provided, Ра.