Kher stands twenty third, writes kh, and carries the numeral 600 from Greek chi. Its shape is the uncial chi, a simple cross of two diagonal strokes, and it is unchanged in the modern alphabets.
The sound is native to Slavonic, which distinguishes this letter from its neighbours fert and fita. Those two exist to write a sound the language did not have, and occur only in borrowed words; kh occurs in ordinary Slavic vocabulary throughout, and every modern Slavic language still has it. The letter is therefore doing two jobs at once: it writes a real Slavonic sound, and it transliterates the Greek letter chi in the Greek words and names the church books are full of.
Its name is the second of the two in the alphabet that nobody has satisfactorily explained. Where azu, buky, vedi, glagol and the rest are transparent Slavonic words, kher is not, and the proposals made for it, from Greek and from Slavonic, have not carried the field. The atlas records that as an open question rather than choosing between them.
As a numeral it is 600, which puts it in the middle of the hundreds series and therefore into a great many dates in Slavonic books, where years are counted from the creation of the world on the Byzantine reckoning and routinely run into the six and seven thousands.