Ra is the second of the four semivowels and it behaves unlike any other consonant in the script. Standing before another consonant it is written as a mark above the syllable rather than as a letter in the line; following one it is written as a mark below. So the same sound appears in two reduced forms as well as in its full one, and neither reduced form resembles the letter.
That makes it one of the first things a reader of Siddham has to learn and one of the commonest sources of error for a learner. A mark above a syllable and a mark below it look like diacritics and are not: they are a consonant, and the syllable they belong to has to be read with the r in the position the mark indicates rather than wherever it falls on the page.
The sound is extremely frequent, both alone and in clusters, and the paired mark above the line is one of the most visible features of a Siddham page.
In East Asian esoteric Buddhism ra is one of the five seed syllables of the elements, where it stands for fire, so it is also among the letters most often cut in stone and written on wooden tablets in Japan, in the third position of the five-tiered stupa.
Because its two reduced forms are marks rather than letters, they are the parts of a syllable most likely to be dropped by a careless copyist or lost to damage on a manuscript. A missing mark above a syllable removes a consonant without leaving a gap in the line, which is a much harder error to notice than a missing character.