Cherv stands twenty sixth, writes the affricate ch, and carries the numeral 90. Its name is the Slavonic word for worm, a word that in the older language also carried the sense of the crimson dye obtained from an insect, which is why the same root gives the modern Slavic words for red.
The sound, like ts, is a product of the palatalisations that reshaped inherited Slavic consonants before front vowels, and it is common in ordinary vocabulary. There was no Greek letter for it and no Greek number attached to any such letter.
The numeral 90 is therefore borrowed rather than inherited. Greek filled that slot with an obsolete sign rather than a letter, and Cyrillic put cherv there, which leaves this letter carrying a lower number than the letter standing before it in the alphabet. That inversion is not a mistake and is explained on the entry for tsy.
The letterform has no Greek model. Its derivation is argued along the same lines as the other signs supplied for Slavonic sounds, and the argument is recorded on the relation to its Glagolitic counterpart.
The letter survives in every modern Cyrillic alphabet, in the same shape and with the same sound, and it is one of the signs that make a page of Cyrillic immediately distinguishable from a page of Greek.