Tsy stands twenty fifth, writes the affricate ts, and carries the numeral 900. Its name is not a word of the language but the sound of the letter with a vowel attached, which is the naming pattern used for most of the signs supplied for sounds Greek did not have.
The sound is thoroughly Slavic. It arose from an inherited k before certain front vowels, in one of the palatalisations that shaped the consonant system of every Slavic language, and its distribution in the grammar still reflects that origin: the alternation between k and ts runs through noun and verb paradigms across the family.
The letterform has no Greek model, and where it came from is argued. The majority reading takes it from the Glagolitic letter of the same sound. A comparison with the Hebrew tsadi has been made since the nineteenth century and is still current, and a third position holds that shapes this simple cannot support either claim. That question is recorded on the relation between this letter and its Glagolitic counterpart rather than settled here.
Its numeral value of 900 is anomalous and deliberately so, and it is set out in the companion article. The letter survives, unchanged in shape and in sound, in every modern Cyrillic alphabet.