Rtsy stands eighteenth, writes r, and carries the numeral 100 from Greek rho. Its name is a Slavonic imperative meaning say or speak, addressed to one person.
Its numeral value is the point at which the Cyrillic and Glagolitic number series converge. Below 100 the two alphabets disagree throughout, because Glagolitic numbers its own letters in their own order while Cyrillic follows the Greek allocation; from 100 upward the two happen to agree letter for letter. That convergence is a small but real piece of evidence in the argument about how the two scripts are related, and it is used by both camps.
The shape is the uncial rho and is unchanged in the modern alphabets. In Slavonic the sound it writes could stand as the centre of a syllable without any vowel, and the earliest orthography wrote a reduced vowel beside it to mark this, which is why words that a modern Slav pronounces with a syllabic r appear in the old books with an extra letter that no longer corresponds to anything spoken.
The letter name, being a command to speak, was a gift to the compilers of primers and of the abecedarian poems, several of which build their line for this letter around the act of proclamation.