The Armenian language most often uses only three words beginning with letter Ր, and all of them are loanwords. These are the words րաբբունապետ ([rabbunapet] – patriarch), the chief of vardapets (archimandrites), Րաֆֆի [Raffi] – the pseudonym of an Armenian writer of XIX century, and րոպե ([rope] – minute), which are defined as exception-words in textbooks of the Armenian language.
In 1615-page Explanatory Dictionary of Modern Armenian Language by Edward Aghayan, which contains 135 000 words, the words beginning with Ր occupy less than half a page. There are about twenty of them, and the majority of these words are paronymous, obsolete, or practically not used. Even more demonstrative in this respect is the 12-volume Armenian Soviet Encyclopedia, which contains only four articles on letter Ր: an article about letter Ր itself is added to the explanation of the above-mentioned three exception-words. By the way, the limited use refers only to the initial position of the letter; in other positions Ր is not uncommon at all.
The Greek letter Р (rho) is believed to be the prototype of the graphic form of letter Ր. The letter was created based on the same principle as a number of other letters – by means of opening of the circle (B – Բ, O – Ո, etc.). The early forms of capital letter Ր have similar inscription, and the small letter – ր – differs primarily in size and position on the line. In the alphabet, letter Ր is after letter Տ [t]: it is quite probable that Mashtots placed them side by side deliberately, because Տ and Ր are the abbreviation of the word Տեր ([ter] – Lord). Besides, the sum of the numerical equivalents of Տ (4000) and Ր (5000) is 9000, which is the numeric value of letter Ք – the symbol of Christ. We have already seen something similar in case of letters Յ [y] and Ն [n], which form the combination Յիսուս Նազովրեցի ([Yisus Nazovretsi] – Jesus of Nazareth), with the total numerical value equal to 700 – the number of so-called hrismon, the monogram of Christ, composed of Greek letters X (Khi) and P (Rho).
See more at Aram Khachaturians "Armenian Alphabet. History and Symbolism" book